Optical Recording Medium and Method for Producing Same

ABSTRACT

It is to obtain an optical recording medium having sufficient stainproof properties and adhesion in a simple and industrially advantageous process. On an optical recording medium comprising a substrate, and a recording and retrieving layer formed on the substrate, a light transmitting layer to be formed on the recording and retrieving layer formed by curing a composition containing silica particles and an oligomer having a urethane bond and capable of being cured by irradiation with radiation, and a stainproof layer to be formed on the light transmitting layer, containing an alkoxysilane compound containing a fluorine atom and/or a hydrolysate of the alkoxysilane compound, are provided.

TECHNICAL FIELD

The present invention relates to an optical recording medium and itsproduction process.

BACKGROUND ART

In an optical recording medium such as next generation DVD employing ablue laser for reading, usually a layer for protection (hereinaftersuitably referred to as “protective layer”) is formed on a recording andretrieving layer so as to protect the recording and retrieving layerwhich performs a recording function. Such a protective layer is requiredto have such properties as a low degree of shrinkage on curing and highhardness and in addition it is required to have stainproof properties toprevent the surface of an optical recording medium from being stained.In order to meet such requirements, heretofore, various developmentshave been conducted regarding technique to form an appropriateprotective layer on the surface of an optical recording medium.

For example, Patent Document 1 discloses a technique of forming on thesurface of an optical recording medium a light transmitting layercontaining a urethane acrylate as the main component, a top layercontaining an active energy ray-curable compound which may containdispersed inorganic component particles as the main component and astainproof layer formed by a light-curable resin containing a fluorinecompound, to make the optical recording medium have stainproofproperties.

Further, Patent Document 2 proposes a composition having a low degree ofshrinkage and a high hardness, employing silica particles and anoligomer having a urethane bond, and it discloses that such acomposition may be employed for a protective layer of an opticalrecording medium.

Patent Document 1: JP-A-2004-83877

Patent Document 2: WO2004/041888

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

However, in the technique as disclosed in Patent Document 1, threelayers are formed on the surface of the optical recording medium, thusthe cost tends to be high, and the production process tends to be poorin producibility and practicability, Further, according to studies bythe present inventors, it was found that when the top layer is to beemployed as a thick anchor layer, the anchor layer may have cracks orthe optical recording medium may have deformations such as warp spring)resulting from shrinkage on curing. Further, it was further found thatadhesion between the top layer and the stainproof layer tends to bepoor.

Further, by the technique as disclosed in Patent Document 2, it isdifficult to increase stainproof properties of the optical recordingmedium.

Under these circumstances, the present invention has been made to solvethe above problems, and it is an object of the present invention toprovide an optical recording medium having sufficient stainproofproperties and adhesion imparted by a simple and industriallyadvantageous method, and its production process.

MEANS OF SOLVING THE PROBLEMS

The present inventors have conducted extensive studies to achieve theabove object and as a result found that an optical recording mediumhaving sufficient stainproof properties and adhesion between a lighttransmitting layer and a stainproof layer imparted by a simple andindustrially advantageous method can be provided by forming on anoptical recording medium having a substrate and a recording andretrieving layer, a light transmitting layer formed by curing acomposition containing silica particles and an oligomer having aurethane bond and capable of being cured by irradiation with radiation,and a stainproof layer containing an alkoxysilane compound containing afluorine atom and/or a hydrolysate of the alkoxysilane compound. Thepresent invention has been accomplished on the basis of this discovery.

Namely, the present invention provides the following:

(1) An optical recording medium which comprises a substrate, a recordingand retrieving layer formed on the substrate, a light transmitting layerformed by curing the following component A, formed on the recording andretrieving layer, and a stainproof layer containing the followingcomponent B, formed on the light transmitting layer:

component A: a composition containing silica particles and an oligomerhaving a urethane bond, capable of being cured by irradiation withradiation,

component B: an alkoxysilane compound containing a fluorine atom and/ora hydrolysate of the alkoxysilane compound.

(2) The optical recording medium according to the above (1) wherein thesilica particles contained in the component A are colloidal silica, orsilica particles of a hydrolysate of an oligomer of an alkoxysilane.

(3) The optical recording medium according to the above (1) or (2)wherein the silica particles contained in the component A have anumber-average particle size of at least 0.5 nm and at most 50 nm.

(4) The optical recording medium according to any one of the above (1)to (3), wherein the silica particles contained in the component A aresurface treated by a silane coupling agent.

(5) The optical recording medium according to any one of the above (1)to (4), wherein the alkoxysilane compound containing a fluorine atomcontained in the component B is a silane coupling agent containing afluoroalkyl group or a fluoroaryl group.

(6) A process for producing the optical recording medium as defined inany one of the above (1) to (5), which comprises a step of curing thecomponent A on the recording and retrieving layer to form the lighttransmitting layer, and a step of applying a composition containing thecomponent B and a solvent and having a solid component in an amount ofat least 0.01 wt. % and at most 1 wt. % to the light transmitting layerand drying the composition to form the stainproof layer.

(7) The process for producing the optical recording medium according tothe above (6), wherein in the step of forming the light transmittinglayer, the silica particles are prepared in a liquid medium containing asolvent, the oligomer having a urethane bond is dissolved in the liquidmedium and the solvent in the liquid medium is removed to prepare thecomponent A.

(8) The process for producing the optical recording medium according tothe above (6) or (7), wherein the solid component includes thealkoxysilane compound containing a fluorine atom and/or the hydrolysateof the alkoxy-silane.

(9) The process for producing the optical recording medium according toany one of the above (6) to (8), wherein the solvent is a halogenorganic solvent.

EFFECTS OF THE INVENTION

According to the present invention, sufficient stainproof properties andadhesion can be imparted to an optical recording medium by a simple andindustrially advantageous method.

Further, according to the process for producing an optical recordingmedium of the present invention, an optical recording medium havingsufficient stainproof properties and adhesion can be securely producedby a simple and industrially advantageous method.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the embodiments of the present invention will be explained below.However, the present invention is by no means restricted to thefollowing examples and the like, and optional modifications are possiblewithin a range not to depart from the scope of the present invention.

The optical recording medium of the present invention comprises asubstrate, a recording and retrieving layer, a light transmitting layerand a stainproof layer. The light transmitting layer is formed by curingthe component A, i.e. a Composition containing silica particles(preferably silica particles made of a hydrolysate of an oligomer of analkoxysilane) and an oligomer having a urethane bond, and capable ofbeing cured by irradiation with radiation (hereinafter suitably referredto as “composition A”). Further, the stainproof layer is formed as onecontaining the component BE i.e. an alkoxysilane compound containing afluorine atom and/or a hydrolysate of the alkoxysilane compound.

(1) Substrate

The substrate is not limited, and a known substrate of an opticalrecording medium may optionally be employed.

A substrate of an optical information recording medium is formed usuallyas a plate having a convex-concave grooves (tracking grooves) to be usedfor recording and retrieving optical information formed on one mainsurface Its shape is optional but it is usually formed into a diskshape.

Further, the material of the substrate is not particularly limited solong as it is a light transmitting material Namely, the substrate can beformed by an optional material through which a light having a wavelengthto be used for recording or retrieving optical information can betransmitted. Specific examples of the material include thermoplasticresins such as a polycarbonate resin, a polymethacrylate resin and apolyolefin resin and glass. Among them, a polycarbonate resin is mostwidely used for e.g. CD-ROM, and is available at a low cost and isthereby most preferred. The materials of the substrate may be used aloneor two or more of them may be used in an optional combination with anoptional proportion.

Further, the dimensions of the substrate are not limited and areoptional. However, the thickness of the substrate is usually at least0.1 mm, preferably at least 0.3 mm, more preferably at least 05 mm, andusually at most 20 mm, preferably at most 15 mm, more preferably at most3 mm. Particularly, a substrate having a thickness at level of 1.2±0.2mm is often used. Further, the outer diameter of the substrate isusually approx. 120 mm.

The method for producing the substrate is not limited and is optional.The substrate may be produced for example, by injection molding of alight transmitting resin employing a stamper.

(2) Recording and Retrieving Layer

The recording and retrieving layer is a layer formed on a substrate soas to have a function of recording and retrieving or retrievinginformation signals. The specific structure of the recording andretrieving layer is not limited, and a known structure as a recordingand retrieving layer of an optical recording medium may optionally beemployed.

The recording and retrieving layer may have a monolayer structureconsisting of only one layer or may have a laminated structureconsisting of a plurality of layers. A layer structure according to thepurpose may be employed, depending upon a case where the opticalrecording medium is a read-only medium (ROM medium), a case where it isa write once medium on which recording is possible only once, and a casewhere it is a rewritable medium on which recording and erasing arerepeatedly carried out.

For example, in a read-only optical recording medium the recording andretrieving layer is usually constituted as a layer having a monolayerstructure consisting of only a reflective layer containing a metal.Further, in such a case, the recording and retrieving layer may beformed, for example, by forming the reflective layer on the substrate bymeans of e.g. a sputtering method.

Further, in a write once optical recording medium, the recording andretrieving layer is usually constituted as a layer having a laminatedstructure prepared by forming a reflective layer and a recording layercontaining an organic dye on a substrate in this order. Further, in sucha case, the recording and retrieving layer may be formed, for example,by forming the reflective layer by means of e.g. a sputtering method,and forming as the recording layer a film of an organic dye on thereflective layer by means of e.g. a spin coating method.

Further, as another specific example of the recording and retrievinglayer in a write once medium, a layer having a laminated structurehaving a reflective layer, a dielectric layer, a recording layer and adielectric layer formed on a substrate in this order may be mentioned.In such a case, usually the dielectric layers and the recording layercontain an inorganic material. Further, such a recording and retrievinglayer in the write once medium may be formed usually by forming thereflective layer, the dielectric layer, the recording layer and thedielectric layer by means of a sputtering method.

Further, in a rewritable optical recording medium, the recording andretrieving layer is usually constituted as a layer having a laminatedstructure having a reflective layer, a dielectric layer, a recordinglayer and a dielectric layer formed on a substrate in this order. Such arecording and retrieving layer in the rewritable medium may be formedusually by forming the reflective layer, the dielectric layer, therecording layer and the dielectric layer by means of a sputteringmethod.

Further, as another specific example of the recording and retrievinglayer in the rewritable optical recording medium, the same recording andretrieving layer as one used for a magneto-optical recording medium maybe mentioned. In such a case, the recording and retrieving layer isformed by a reflective layer, a recording layer and a dielectric layer.

Further, in the optical recording medium, a recording and retrievingregion to be used for practical recording and retrieving is usually set.This recording and retrieving region is provided usually at a regionhaving an inner diameter larger than the inner diameter of the recordingand retrieving layer and having an outer diameter smaller than the outerdiameter of the recording and retrieving layer. At this recording andretrieving region, the above tracking grooves are formed on thesubstrate.

Now, the layers constituting the recording and retrieving layer will beexplained in detail below.

(2-1) Reflective Layer

The reflective layer is a layer on which a light to be employed forrecording and retrieving is reflected in the optical recording medium.Its specific structure is not limited, and a known reflective layer ofan optical recording medium may optionally be employed.

As a material to be used for the reflective layer, an optional materialmay be employed so long as a light to be employed for recording andretrieving is reflected, but usually a substance having a highreflectivity is preferred. Further, the materials of the reflectivelayer may be used alone or two or more of them may be used in anoptional combination with an optional proportion.

As examples of a preferred material of the reflective layer, metals suchas Au, Ag and Al may be mentioned, with which heat dispersion effectsare expected.

Further, in order to control the thermal conductivity of the reflectivelayer itself or to improve corrosion resistance, a metal such as Ta, Ti,Cr, Mo, Mg, V, Nb, Zr or Si may be used in combination. The amount of ametal used in combination is usually at least 0.01 mol. % and at most 20mol. % as a proportion in the reflective layer.

Particularly, an aluminum alloy containing Ta and/or Ti in an amount ofat most 15 mol. %, particularly an aluminum alloy of Al_(α)Ta_((1-α))(wherein 0≦α≦0.15) is excellent in corrosion resistance and isparticularly preferred with a view to improving reliability of theoptical recording medium.

Further, a Ag alloy containing Ag and at least one of Mg, Ti, Au, Cu,Pd, Pt, Zn, Cr, Si, Ge and rare earth elements in an amount of at least0.01 mol. % and at most 10 mol. % has a high refractivity and a highthermal conductivity and is excellent also in heat resistance and isthereby preferred.

Further, the thickness of the reflective layer is not limited and isoptional, and it is usually at least 40 nm, preferably at least 50 nmand usually at most 300 nm, preferably at most 200 nm. If the reflectivelayer is excessively thick, the shape of the tracking grooves formed onthe substrate may change, and further, film formation will take long,and the material cost tends to increase. On the other hand, if thereflective layer is excessively thin, not only light transmission willtake place and the layer may not function as a reflective layer but alsoa part of the reflective layer is likely to be influenced by an islandstructure formed at the initial stage of the film growth, whereby thereflectivity or the thermal conductivity may decrease.

(2-2) Dielectric Layer

The dielectric layer is a layer to prevent evaporation or deformationaccompanying a phase change of the recording layer, and to controlthermal diffusion at the time of the phase change. Its specificstructure is not limited, and a known dielectric layer of an opticalrecording medium may optionally be employed.

As a material to be used for the dielectric layer, an optional materialmay be employed so long as it is a dielectric, and usually it ispreferably selected considering refractivity, thermal conductivity,chemical stability, mechanical strength, adhesion, etc. Usually, it ispossible to employ a dielectric material having high transparency andhaving a high melting point, such as an oxide, a sulfide, a nitride or acarbide of a metal or a semiconductor, or a fluoride of e.g. Ca Mg orLi. Further, the materials of the dielectric layer may be used alone ortwo or more of them may be used in an optional combination with anoptional proportion. Further, the above materials such as an oxide, asulfide, a nitride, a carbide and a fluoride are not necessarilyrequired to have a stoichiometrical composition, and it is effective tocontrol their compositions or to mix them so as to control therefractivity etc Specifically, the material of the dielectric layer may,for example, be an oxide of a metal such as Sc, Y, Ce, La, Ti, Zr, Hf,V, Nb, Ta, Zn, Al, Cr, In, Si, Ge, Sn, Sb or Te; a nitride of a metalsuch as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Zn, B, Al, Ga, In, Si, Ge, Sn,Sb or Pb; a carbide of a metal such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W,Zn, B, Al, Ga, In or Si or a mixture thereof. Further, a sulfide of ametal such as Zn, X, Cd, Ga, In, Si, Ge, Sn, Pb, Sb or Bi; a selenide ortelluride; a fluoride of e.g. Ma or Ca, or a mixture thereof, may alsobe mentioned.

Among them, considering repeated recording characteristics of theoptical recording medium, it is preferred to form the dielectric layerby a mixture of dielectrics. For example, a dielectric layer may beformed by a mixture of a chalcogenide of e.g. ZnS or a rare earthsulfide with a heat resistant compound such as an oxide, a nitride, acarbide or a fluoride. Further, a mixture of heat resistant compoundscontaining ZnS as the main component or a mixture of heat resistantcompounds containing a rare earth oxysulfide particularly Y₂O₂S as themain component, is one example of a preferred dielectric layercomposition. More specifically, ZnS—SiO₂ SiN, SiO₂, TiO₂, CrN, TaS₂ orY₂O₂S may, for example, be mentioned. Among these materials, ZnS—SiO₂ iswidely utilized in view of a high film formation rate, a small filmstress, a small volume change by a temperature change and excellentweather resistance.

Further, the thickness of the dielectric layer is not limited and isoptional, and it is usually at least 1 nm, and usually at most 500 nm.When its thickness is at least 1 nm, a sufficient effect of preventingdeformation of the substrate or the recording layer will be secured, andthe layer can sufficiently play a role as a dielectric layer. Further,when the thickness is at most 500 nm, while the layer can sufficientlyplay a role as a dielectric layer, cracks can be prevented which are dueto a remarkable internal stress of the dielectric layer itself and aremarkable change of the elastic characteristics with the substrate.

(2-3) Recording Layer

The recording layer is a layer on which information is recorded by itsphase change. Its specific structure is not limited, and a knownrecording layer of an optical recording medium may optionally beemployed.

As the material to be used for the recording layer, known materials mayoptionally be employed, and the materials may be used alone or two ormore of them may be used in an optional combination with an optionalproportion.

Specifically, the material may be a compound having a composition ofe.g. GeSbTe, InSbTe, AgSbTe or AgInSbTe. Among them, a thin filmcontaining as the main component a{(Sb₂Te₃)_((1-x))(GeTe)_(x)}_((1-y))Sb_(y) alloy (wherein 0.2≦x≦0.9 and0≦y≦0.1) or a {Sb_(x)Te_((1-x))}_(y)M_((1-y)) alloy (wherein 0.6≦x≦0.9,0.7≦y≦1, and M is at least one atom selected from Ge, Ag, In, Ga, Zn,Sn, Si, Cu, Au, Pd, Pt, Pb, Cr, Co, O, S, Se, V, Nb and Ta) is preferredsince it is stable in either crystallized or amorphous state and iscapable of undergoing phase change (phase transition) between thesestates at a high speed. Further, such a material has an advantage suchthat segregation hardly occurs when repeated overwriting is carried out,and is the most practical material.

Further, as a material of the recording layer, an organic dye may beused instead of or in combination with the above inorganic compound.Specifically, the organic dye may, for example, be a macrocyclicazaannulene dye (such as phthalocyanine dye, naphthalocyanine dye orporphyrin dye), a pyromethene dye, a polymethine dye (such as cyaninedye, merocyanine dye or squarylium dye) an anthraquinone dye, anazulenium dye, a metal complex of azo dyes or a metal complex ofindoaniline dyes. Among these organic dyes, a metal complex of azo dyesis excellent in recording sensitivity and is excellent also indurability and light resistance and is thereby preferred.

Further, the organic dye to be used for the recording layer ispreferably a dye compound having a maximum absorption wavelength λmax ina visible light to near infrared region at a wavelength of about from350 to about 900 nm and suitable for recording by means of a blue tonear microwave laser. More preferred is a dye suitable for recording bymeans of a near infrared laser having a wavelength at a level of from770 to 830 nm usually used for CD-R (typically 780 nm, 830 nm) a redlaser having a wavelength at a level of from 620 to 690 nm (typically,635 nm, 650 nm, 680 nm, etc.) or a so-called blue laser having awavelength of 410 nm, 515 nm, etc.

Further, the thickness of the recording layer is not limited and isoptional, but the lower limit is usually at least 5 nm, preferably atleast 10 nm. Within such a range, a sufficient optical contrast betweenan amorphous state and a crystallized state of the recording layer willbe obtained. Further, the upper limit of the thickness of the recordinglayer is usually at most 30 nm, preferably at most 20 nm. Within such arange, an increase in the optical contrast due to reflection of a lighttransmitted through the recording layer on the reflective layer will beobtained, and the heat capacity can be controlled to be a proper value,whereby high speed recording becomes possible.

Further, when the thickness of the recording layer is at least 10 nm andat most 20 mL both higher speed recording and higher optical contrastcan be achieved at the same time. Further, when the thickness of therecording layer is within this range, it is possible to reduce thevolume change due to a phase change and to minimize the influence of therepeated volume change due to repetitive overwriting, over the recordinglayer itself and over other layers on and below the recording layer.Further, accumulation of irreversible microscopic deformation of therecording layer can be suppressed, whereby the noise will be reduced,and the repetitive overwriting durability will be improved

(2-4) Others

The layers constituting the recording and retrieving layer such as thereflective layer, the recording layer and the dielectric layer may beformed by an optional method, and they are formed usually by e.g. asputtering method. In the sputtering method, it is preferred to carryout layer formation in an in-line apparatus having a recording layertarget, a dielectric layer target, if necessary a reflective layermaterial target provided in the same vacuum chamber, with a view topreventing oxidation or contamination among the layers. Such isexcellent also in view of productivity. Further, in a case of forminglayers by e.g. an organic dye, the layer formation may be carried oute.g. by a spin coating method.

On the optical recording medium of the present invention the lighttransmitting layer and the stainproof layer are formed on a mediumhaving the above-described substrate and recording and retrieving layer,and among such recording media, a next generation high density opticalrecording medium employing a blue laser is preferred. Accordingly,although the wavelength of a light to be employed for recording andretrieving on the optical recording medium of the present invention isnot limited and is optional, it is preferred to form the medium as anoptical recording medium employing a light having a wavelength ofusually at least 350 nm, preferably at least 380 nm, and usually at most800 nm, preferably at most 450 nm.

(3) Light Transmitting Layer

The light transmitting layer is a layer to be formed on the recordingand retrieving layer for the purpose of protecting the recording andretrieving layer or for another purpose. Further, in the opticalrecording medium of the present invention, the light transmitting layeris formed by curing the composition A.

The composition A is a composition containing silica particles(hereinafter suitably referred to as “fine silica” in some cases) and anoligomer having a urethane bond (hereinafter suitably referred to as“urethane oligomer” in some case), and capable of being cured byirradiation with radiation. Further, in the composition A, anotherinorganic component (hereinafter suitably referred to as “combinedinorganic component” in some cases) may suitably be incorporated.

(3-1) Fine Silica Particles

The particle size of the fine silica particles contained in thecomposition A is optional within a range not to significantly impair theeffects of the present invention. However, the lower limit of thenumber-average particle size of the fine silica particles of thecomposition A is usually at least 0.5 nm, preferably at least 1 nm. Ifthe number-average particle size is too small, aggregation properties ofthe fine silica particles which are ultrafine particles tend toextremely increase, whereby when the composition A is cured, the curedcomposition A i.e. the light transmitting layer may have extremelydecreased transparency or mechanical strength, or characteristics by thequantum effect may not be remarkable. Further, the upper limit of thenumber-average particle size of the fine silica particles of thecomposition A is usually at most 50 nm, preferably at most 40 nm, morepreferably at most 30 nm, furthermore preferably at most 15 nm,particularly preferably at most 12 nm.

Further, in the composition A, the proportion of the fine silicaparticles having predetermined particle sizes is preferably within apredetermined range. Specifically, among the fine silica particles ofthe composition A, the proportion of fine silica particles havingparticle sizes usually larger than 30 nm, preferably larger than 15 nm,is desirably usually at most 1 wt. %, preferably at most 0.5 wt. % basedon the composition A. Otherwise, the proportion of the fine silicaparticles having particles sizes within the above range is desirablyusually at most 1 vol. %, preferably at most 0.5 vol % based on thelight transmitting layer. If the composition A contains the fine silicaparticles having particle sizes within the above predetermined range ina large amount, light scattering tends to be significant, whereby thetransmittance tends to decrease.

To determine the above number-average particle size, values as measuredin an image observed by a transmission electron microscope (TEM) areemployed. Namely, the diameter of a circle having the same area as thatof an observed fine silica particle is defined as the particle size ofthe silica particle. Employing particle sizes thus determined, the abovenumber-average particle size is calculated by means of known statisticalanalysis of image data. On this occasion, the number of the ultrafineparticle images to be used for the statistical analysis (statisticanalysis data number) is preferably as many as possible. For example,the number of randomly selected fine silica particle images is usuallyat least 50 pieces, preferably at least 80 pieces, more preferably atleast 100 pieces in view of reproducibility. Further, the vol. % of thefine silica particles in the cured composition A is calculated as thevolume of spheres having the above-measured particle sizes as diameters.

Conventional silica particles usually provide a broad particle sizedistribution, and include particles having particle sizes larger than 50nm for example. Accordingly, their transparency tends to be poor in manycases, and the silica particles are likely to sediment. A product fromwhich silica particles having large particles sizes are separated (aso-called cut product) is known, but such silica particles tend toundergo secondary aggregation, and their transparency are impaired inmost cases.

On the other hand, the composition A contains fine silica particles.

Such fine silica particles are not particularly limited, and colloidalsilica or silica particles of a hydrolysate of an oligomer of analkoxysilane may, for example, be mentioned.

First, colloidal silica will be explained below. Colloidal silica isusually one in such a state that ultrafine particles of silicicanhydride are dispersed in water or an organic solvent. The primaryparticle size of the colloidal silica is usually at least 1 nm,preferably at least 5 nm, and usually at most 200 nm, preferably at most80 nm. If the primary particle size of the colloidal silica is smallerthan the lower limit of the above range, the silica component tend togelate during preservation or in the production process, and if theprimary particle size is larger than the upper limit, transparency ofthe light transmitting layer tends to decrease.

Further, as specific examples of a dispersion medium to be used todisperse the ultrafine particles of silicic anhydride, known dispersionmedia may optionally be employed, and the dispersion medium may, forexample, be water; an alcohol solvent such as methanol, ethanol,2-propanol, n-propanol, 2-butanol or n-butanol; a polyhydric alcoholsolvent such as ethylene glycol; a polyhydric alcohol derivative such asethyl cellosolve or butyl cellosolve; a ketone solvent such as methylethyl ketone, methyl isobutyl ketone or diacetone alcohol; or a monomersuch as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate ortetrahydrofurfuryl acrylate. Among them, an alcohol solvent having threeor less carbon atoms is particularly preferred. The above dispersionmedia may be used alone or two or more of them may be used in anoptional combination with an optional proportion. Further, the colloidalsilica may be produced by a known method, or may be available as acommercial product.

Now, the silica particles of a hydrolysate of an oligomer of analkoxysilane will be described below. According to a specificpreparation method of hydrolysis of an oligomer of an alkoxysilane, finesilica particles having very small particle sizes will stably beobtained, and such fine silica particles has such characteristics thatthey are hardly aggregated. Accordingly, when the silica particlesobtained by this preparation method are employed as the fine silicaparticles of the composition A, high transparency will be achieved evenwhen the composition A is cured.

The hydrolysate means a product obtained by reactions including at leasta hydrolytic reaction, and the reaction may involve dehydrationcondensation for example. Further, the hydrolytic reaction includesdealcoholization.

The alkoxysilane is a compound having an alkoxy group bonded to asilicon atom and this forms an alkoxysilane multimer (oligomer) by ahydrolytic reaction or a dehydration condensation reaction (ordealcoholization condensation). The type of the alkoxysilane to beemployed for the material of the fine silica particles of thecomposition A is not limited and is optional, but the number of carbonatoms in the alkyl group of the alkoxysilane is preferably not too manyand it is usually at least 1, and usually at most 5, preferably at most3, in order that the alkoxysilane oligomer has compatibility with wateror a solvent as described hereinafter. Specifically the alkoxysilane tobe used as the material of the fine silica particles of the compositionA may, for example, be tetramethoxysilane or tetraethoxysilane.

Further, the fine silica particles of the composition A are obtainedusually from an oligomer of the above alkoxysilane as a startingmaterial. The reasons why the alkoxysilane monomer is not used as astarting material are as follows. Namely, it tends to be difficult tocontrol the particle size of the alkoxysilane monomer, and thedistribution of the particle sizes tends to be broad, and the particlesizes are hardly uniform, whereby it may be impossible to make thecomposition A transparent. Further, some of alkoxysilane monomers havetoxicity, and such is unfavorable in view of safety and sanitation.

As the oligomer of the alkoxysilane, an alkoxysilane oligomer producedby an optional known production method may be employed, and it can beproduced, for example, by a method as disclosed in JP-A-7-48454.

Further, the alkoxysilane oligomer to be employed as the material of thefine silica particles of the composition A preferably has compatibilitywith a solvent or water to be used at the time of hydrolysis. This is toprevent phase separation in a hydrolysis step.

The molecular weight of the alkoxysilane oligomer to be employed as thematerial of the fine silica particles is not limited and is optionalwithin a range not to significantly impair the effects of the presentinvention. However, it is usually at least 100, preferably at least 200,more preferably at least 300, and usually at most 1,500, preferably atmost 1,200, more preferably at most 1,000. If the molecular weight isout of this range, white turbidity or gelation is likely to occur whenthe fine silica particles are formed.

The alkoxysilane oligomers as the material may be used alone or two ormore of them may be used in an optional combination with an optionalproportion.

The method of hydrolysis of the alkoxysilane oligomer is not limited andis optional and for example, the hydrolysis can be carried out by addinga certain amount of water to the alkoxysilane oligomer in a specificsolvent and reacting a catalyst. The fine silica particles of thecomposition A can be obtained by such a hydrolytic reaction.

The solvent to be employed for the hydrolysis is optional so long ashydrolysis of the alkoxysilane oligomer is possible, and it is possibleto use one or at least two in combination among alcohols, glycolderivatives, hydrocarbons, esters, ketones and ethers. Among them,alcohols and ketones are particularly preferred.

Specific examples of the alcohols include methanol, ethanol, isopropylalcohol n-butyl alcohol, isobutyl alcohol, octanol, n-propyl alcohol andacetylacetone alcohol.

Further, specific examples of the ketones include acetone, methyl ethylketone and methyl isobutyl ketone.

Further, in order that the fine silica particles which are hydrophilicare stably present, the number of carbon atoms in such an alcohol orketone is preferably small. Particularly preferred is methanol, ethanolor acetone. Particularly, acetone, which has a low boiling point, hassuch an advantage that the time required for a step of removing thesolvent is relatively short.

The amount of water to be used for the hydrolytic reaction of thealkoxysilane oligomer is not limited and is optional so long ashydrolysis is possible. However, it is preferred to use water in anamount of usually at least 0.05 time by mol, preferably at least 0.3time by mol, to the number of mol, of the alkoxy groups which thealkoxysilane oligomer has. If the amount of water is too small, the finesilica particles will not grow to have a sufficient size, whereby nodesired characteristics of the fine silica particles tend to beobtained. However, the upper limit is usually at most one time. If theamount is too large, the alkoxysilane oligomer tends to form a gel.

Further, the catalyst to be used for the hydrolysis is not limited, andan optional catalyst may be employed so long as the hydrolysis ispossible. For example, a metal chelate compound, an organic acid, ametal alkoxide or a boron compound may be employed. Among them, a metalchelate compound or an organic acid is preferred. The catalysts to beused for the hydrolysis may be used alone or two or more of them may beused in an optional combination with an optional proportion.

Specifically, the metal chelate compound to be used as the catalyst may,for example, be aluminum tris(acetylacetonate), titaniumtetrakis(acetylacetonate), titanium bis(isopropoxy)bis(acetylacetonate),zirconium tetrakis(acetylacetonate), zirconiumbis(butoxy)bis(acetylacetonate) or zirconiumbis(isopropoxy)bis(acetylacetonate), and they may be used alone or incombination of two or more of them Especially, aluminumtris(acetylacetonate) is preferably employed.

Further, specific examples of the organic acid to be used as thecatalyst include formic acid, acetic acid, propionic acid and maleicacid, and they may be used alone or in combination of two or more ofthem. Especially, maleic acid is preferably employed. In a case wheremaleic acid is employed, such an advantage is obtained that the lighttransmitting layer obtained by curing the composition A with radiationtends to have favorable hue and is less likely to be yellowish.

The amount of such a catalytic component is not particularly limitedwithin a range where its effect is sufficiently achieved and it ispreferably usually at least 0.1 part by weight, preferably at least 0.5part by weight per 100 parts by weight of the alkoxysilane oligomer.However, it is usually at most 10 parts by weight, preferably at most 5parts by weight, since the effect will not longer improve even if a verylarge amount of the catalyst is used.

The temperature condition when the hydrolysis is carried out is notlimited and is optional so long as the hydrolysis proceeds, and thetemperature is desirably usually at least 10° C., preferably 30° C., andusually at most 90° C., preferably at most 70° C. If the temperature islower than the lower limit of this range, the reaction to form the finesilica particles may not sufficiently proceed, and if it is higher thanthe upper limit, gelation of the oligomer of the alkoxysilane may easilyoccur.

Further, the hydrolysis time during which the hydrolysis is carried outis not limited, and it is usually from 30 minutes to 1 week.

In the present invention, by use of the fine silica particles of ahydrolysate of an oligomer of an alkoxysilane as the silica particles tobe employed for the composition A, such an advantage will be obtainedthat very small ultrafine particles having a much uniform particle sizecan be incorporated in the light transmitting layer as compared withsilica particles which are commonly employed as a filler component.Further, since the fine silica particles of a hydrolysate of analkoxysilane oligomer have such characteristics that they hardlyaggregate, the fine silica particles can be uniformly dispersed in thecomposition A, which makes it possible to uniformly disperse the finesilica particles in the light transmitting layer. Accordingly, theradiation transmittance will not be impaired even when a large amount ofthe fine silica particles are used and thus the fine silica particles inan amount sufficient to increase the dimensional stability and themechanical strength of the light transmitting layer and the opticalrecording medium can be used. By use of the fine silica particlesobtained by such a specific production method and a surface treatment ofthe fine silica particles by means of a silane coupling agent or thelike as described hereinafter in combination, and by further employing aurethane oligomer, such an advantage can be obtained that a largeramount of the fine silica particles can be dispersed withoutaggregation.

Accordingly, a light transmitting layer employing the above fine silicaparticles has such an advantage that it has excellent characteristics intransparency dimensional stability mechanical strength and adhesion etc

(3-2) Combined Inorganic Component

In the composition A, in addition to the fine silica particles, anotherinorganic component (combined inorganic component) may be incorporated.The combined inorganic component is not particularly limited and anoptional inorganic substance may be employed within a range not tosignificantly impair the effects of the present invention. The combinedinorganic component may, for example, be a colorless metal or acolorless metal oxide. Specifically it may, for example, be silver,palladium alumina, zirconia, aluminum hydroxide, titanium oxide, zincoxide, silica particles other than the above described fine silicaparticles, calcium carbonate or a clay mineral powder. Among them,preferred is alumina, zinc oxide, silica particles other than the finesilica particles or titanium oxide. Such combined inorganic componentsmay be used alone or two or more of them may be used in an optionalcombination with an optional proportion.

The method for producing the combined inorganic component is not limitedand an optional method may be employed. However, since the combinedinorganic component preferably has a small particle size, it ispreferably produced by means of a method capable of reducing theparticle size of the particles. Specifically a method of pulverizing acommercial combined inorganic component product by a pulverizer such asa ball mill; or a method of producing the combined inorganic componentby a sol-gel method, may, or example be mentioned. Among them, preferredis a product produced by a sol-gel method.

The particle size of the combined inorganic component to be incorporatedin the composition A is optional so long as the effects of the presentinvention are not significantly impaired and usually as mentioned above,the combined inorganic component is preferably in the form of ultrafineparticles having a small particle size. The lower limit of the particlesize of the combined inorganic component is usually at least 0.5 nm,preferably at least 1 nm as the number-average particle size. If thenumber-average particle size is too small, aggregation properties of theultrafine particles tend to extremely increase, whereby the transparencyor the mechanical strength of the light transmitting layer may extremelydecrease, or the characteristics by the quantum effect may not beremarkable. The upper limit of the particle size of the combinedinorganic component is usually at most 50 nm, preferably at most 40 nm,more preferably 30 nm, furthermore preferably less than 15 nmparticularly preferably at most 12 nm as the number-average particlesize.

Further, in the composition A, the proportion of the combined inorganiccomponent having predetermined particle sizes is preferably within apredetermined range, in the same manner as in the case of the finesilica particles. Specifically, among the combined inorganic componentsof the composition A, the proportion of the combined inorganic componenthaving particle sizes usually larger than 30 nm, preferably larger than15 nm, desirably usually at most 1 wt. %, preferably at most 0.5 wt. %,based on the composition A. Otherwise, the proportion of the combinedinorganic component having particle sizes within the same range, isdesirably usually at most 1 vol. %, preferably at most 0.5 vol. %, basedon the light transmitting layer. If the composition A contains thecombined inorganic component having particle sizes within the abovepredetermined range in a large amount, light scattering tends to besignificant, whereby the transmittance tends to decrease. Thenumber-average particle size can be determined in the same manner asdescribed above.

(3-3) Composition of Inorganic Component

With respect to the total inorganic component content of the fine silicaparticles and the combined inorganic component (hereinafter suitablyreferred to as “dispersive inorganic components”) in the composition A,they are incorporated in an amount as large as possible so as toincrease the dimensional stability and the hardness characteristics ofthe light transmitting layer. Specifically, it is desirable toincorporate the dispersive inorganic components in an amount of usuallyat least 5 wt. %, preferably at least 10 wt. % based on the compositionA. Otherwise, it is desirable to incorporate the dispersive inorganiccomponents in an amount of usually at least 2 vol. %, preferably atleast 5 vol. %, based on the light transmitting layer.

However, the content is preferably not too high so as to maintain hightransparency and mechanical strength of the light transmitting layer,and the content of the dispersive inorganic components in thecomposition A is desirably usually at most 60 wt. %, preferably at most40 wt. %, furthermore preferably at most 30 wt. %. Otherwise, thecontent of the dispersive inorganic components in the light transmittinglayer is desirably usually at most 30 vol. %, preferably at most 20 vol.%, furthermore preferably at most 15 vol. %.

Further, the proportion of the fine silica particles to the dispersiveinorganic components is not particularly limited and is optional, and itis usually at least 50 wt %, preferably at least 60 wt. %, furthermorepreferably at least 70 wt. % and usually at most 100 wt. %.

(3-4) Surface Treatment

It is preferred to protect the surface of the particles of thedispersive inorganic components including the fine silica particles bysurface treatment, if necessary.

Usually, the above dispersive inorganic components particularly the finesilica particles in the composition A formed as described above havehigh polarity and thereby have compatibility with water, an alcohol,etc. and have no compatibility with a urethane oligomer in some cases.Accordingly when they are dispersed in a urethane oligomer, they mayundergo aggregation or they may cause white turbidity.

Accordingly, the surface of the particles of the dispersive inorganiccomponents is made to be hydrophobic by applying a surface treatingagent to the dispersive inorganic components, whereby the dispersiveinorganic components are made to have compatibility with a urethaneoligomer, so as to prevent aggregation and white turbidity. As thesurface treating agent, for example, one having a hydrophilic functionalgroup and a hydrophobic functional group may be employed andspecifically, a dispersing agent, a surfactant or a coupling agent, may,for example, be employed. Further, the surface treating agents may beused alone or two or more of them may be used in an optional combinationwith an optional proportion.

The method of the surface treatment is not limited so long as the aboveaggregation and white turbidity can be prevented and use of the abovedispersing agent or surfactant, or a method of modifying the surface bymeans of e.g. a coupling agent, may, for example, be preferablyemployed.

As the dispersing agent, known one may optionally be employed, and itmay be selected from polymer dispersing agents to be used fordispersions of fine particles such as various inks, coatings and tonersfor electrophotographes. Such a polymer dispersing agent is suitablyselected from acrylic polymer dispersing agent urethane polymerdispersing agents and the like. Specifically, EFKA trade name,manufactured by EFKA ADDITIVES, Disperbyk, trade name, manufactured byBYK-Chemie and DISPALON, trade name, manufactured by Kusumoto Chemicals,Ltd. may, for example, be mentioned.

Further, the amount of the dispersing agent to be used is optional, andit is usually at least 10 wt. %, preferably at least 20 wt. %, andusually at most 500 wt. %, preferably at most 300 wt. %, based on thedispersive inorganic components.

Further, as the surfactant, known surfactants may optionally beemployed, and it may be selected from polymer or low molecular weightcationic, anionic, nonionic and amphoteric non-aqueous surfactants.Specifically, it may, for example, be a sulfonic acid amid surfactant(“Solsperse 3000”, manufactured by Avecia Pigments & Additives), ahydrostearic acid surfactant (“Solsperse 17000”, manufactured by AveciaPigments & Additives), an aliphatic amine surfactant, an ε-caprolactonesurfactant (“Solsperse 24000”, manufactured by Avecia Pigments &Additives), a 1,2-hydroxystearic acid multimer or tallow diamine oleate(“Duomeen TDO”, manufactured by LION AKZO CO., LTD.).

Further, the amount of the surfactant to be used is optional, and it isusually at least 10 wt. %, preferably at least 20 wt. %, and usually atmost 500 wt %, preferably at most 300 wt. %, based on the dispersiveinorganic components.

Further, among the dispersive inorganic components, particularly thefine silica particles are preferably subjected to surface treatment by asilane coupling agent. The silane coupling agent is a compound havingsuch a structure that an alkoxy group and an alkyl group having afunctional group are bonded to a silicon atom, and has a function tomake the surface of the silica particles be hydrophobic. Namely, in acase of carrying out the surface treatment of the fine silica particlesemploying a silane coupling agent, a dealcoholization reaction takesplace between the alkoxy group of the silane coupling agent and thehydroxyl group on the surface of the fine silica particles, whereby aSi—O—Si bond will be formed.

The silane coupling agent is not particularly limited and is optional solong as the above object is achieved, and particularly preferred is atrialkoxysilane having a radiation-curable functional group.Specifically, it may, for example, beepoxycyclohexylethyltrimethoxysilane, glycid oxypropyltrimethoxysilane,vinyltrimethoxysilane vinyltriethoxysilane,acryloxypropyltrimethoxysilane methacryloxypropyltrimethoxysilane,mercaptopropyltrimethoxysilane or mercaptopropyltriethoxysilane.

Further, the amount of the silane coupling agent to be used is optionalso long as the above aggregation and white turbidity can be prevented,and it is desirably usually at least 1 wt. %, preferably at least 3 wt.%, more preferably at least 5 wt. % based on the fine silica particles.If the amount of the silane coupling agent is too small, the surface ofthe fine silica particles may not sufficiently be hydrophobic, anduniform mixing with a urethane oligomer may be impaired. On the otherhand, if the amount is too large, the silane coupling agent which willnot be bonded to the fine silica particles will be contained in thecomposition A in a large amount, whereby the transparency, themechanical properties, etc. of the light transmitting layer to beobtained tend to be impaired. Accordingly, the upper limit of the amountof the silane coupling agent to be used is usually at most 400 wt. %,preferably at most 350 wt. %, more preferably at most 300 wt. %.

Further, the silane coupling agent may be partially hydrolyzed at thetime of the surface treatment in some cases. Accordingly, when the finesilica particles are subjected to the surface treatment by the silanecoupling agent, the composition A thus obtained may contain fine silicaparticles which are surface treated by a compound selected from thegroup consisting of the silane coupling agent, a hydrolyzate of thesilane coupling agent and a condensate thereof in some cases. Further,it may contain a condensate of the silane coupling agent and/or acondensate of the silane coupling agent with its hydrolysate in somecases.

Here, the hydrolysate of the silane coupling agent means a productwherein some of or all the alkoxysilane groups of the silane couplingagent are converted into silanol groups via the hydrolytic reaction anda part of or the entire silane coupling agent is converted into ahydroxysilane. For example, in a case where the silane coupling agent isepoxycyclohexylethyltrimethoxysilane the hydrolysate includesepoxycyclohexylethylhydroxydimethoxysilane,epoxycyclohexylethyldihydroxymethoxysilane andepoxycyclohexylethyltrihydroxysilane.

Further, the condensate of the silane coupling agent and/or thecondensate of the silane coupling agent and its hydrolysate means aproduct wherein the alkoxyl group undergoes a dealcoholization reactionwith the silanol group to form a Si—O—Si bond, or the silanol groupundergoes a dehydration reaction with another silanol group to form aSi—O—Si bond.

The above-described surface treating agent is employed in a methoddepending upon the type of the surface treating agent or the purpose ofuse, to carry out the surface treatment of the dispersive inorganiccomponents.

For example, when the surfactant or the dispersing agent is employed asthe surface treating agent, a method of mixing a liquid medium havingthe fine silica particles dispersed therein with the surface treatingagent, followed by stirring at a temperature of from room temperature to60° C. for from 30 minutes to 2 hours for a reaction or a method ofmixing them for a reaction, followed by aging at room temperature forseveral days, may, for example, be mentioned. In the case of mixing itis preferred not to select, as the liquid medium, a solvent in which thesurface treating agent is very highly soluble. In a case where such asolvent in which the surface treating agent is very soluble is employed,the dispersive inorganic components may not sufficiently be protected,or the protection process requires a very long time. If such a solventin which the surface treating agent is very highly soluble is used asthe liquid medium, the dispersive inorganic components will besufficiently protected in many cases by employing, for example, asolvent which provides a difference in solubility (SP value) between thesolvent and the surface treating agent of at least 0.5.

Further, in a case where the silane coupling agent is employed as thesurface treating agent for example, usually the surface treatmentreaction is made to proceed at room temperature (25° C.). Usually thereaction is made to proceed with stirring for from 0.5 to 24 hours, andit may be carried out under heating at a temperature of not higher than100° C. By heating, the reaction rate tends to increase, and thereaction can be carried out in a shorter time. Further, when the silanecoupling agent is employed, water may be mixed. It is preferred to mixwater usually in an amount required for hydrolysis of the alkoxy groupsderived from the silane coupling agent and the remaining alkoxy groupderived from the alkoxysilane.

Further, the silane coupling agent may be mixed all at once, or may bemixed dividedly in two or more times. When the silane coupling agent isdividedly mixed in two or more times, water may also be dividedly mixedin two or more times, and in such a case, the amount is the same as theabove-explained amount of use of water for the silane coupling agent.

Here, in a case where the surface treatment is carried out by employingthe silane coupling agent, it is preferred to mix the dispersiveinorganic components and the urethane oligomer after sufficientcompletion of the surface treatment before the dispersive inorganiccomponents and the urethane oligomer are mixed. If the dispersiveinorganic components and the urethane oligomer are mixed beforesufficient progress of the surface treatment, the urethane oligomer maynot uniformly be mixed, or the composition A may undergo white turbidityin the subsequent steps. Conformation whether the surface treatmentemploying the silane coupling agent is sufficiently completed may becarried out by measuring the amount of remaining silane coupling agentin the reaction liquid under the surface treatment. Usually, sufficientcompletion of the reaction in the surface treatment step can beconfirmed when the amount of the remaining silane coupling agent in thereaction liquid becomes 10% or less of the charged amount.

(3-5) Urethane Oligomer

The urethane oligomer contained in the composition A is not particularlylimited so long as it is an oligomer of an organic compound having aurethane bond, and an optional oligomer may be employed. Incorporationof the urethane oligomer in the composition A provides such an advantagethat adhesion and the degree of surface cure of the light transmittinglayer to be formed by curing the composition A tend to increase.

Such a phenomenon that the adhesion of the light transmitting layer toan adherend (usually the recording and retrieving layer) improves whenthe urethane oligomer is employed, is considered to be attributable toincreased interaction with the adherend by electrical polarity of theurethane bond.

Further, the reason why the degree of surface cure improves when theurethane oligomer is employed is not clearly understood, but the majorreason is estimated as follows Namely, in the composition A containingthe urethane oligomer in a certain amount or more, intramolecularhydrogen bonds or intermolecular hydrogen bonds due to electric polarityof the urethane bond are likely to form, whereby aggregation propertiesof the oligomer tend to increase, and resultingly free movement ofoxygen in the composition A is impaired, and radical polymerizationinhibition is suppressed.

Further, the urethane oligomer is preferably such that the urethaneoligomer itself has a radiation-curable functional group also, wherebythe urethane oligomer is incorporated and united in theradiation-curable network structure. Accordingly, when the composition Ais cured by irradiation with radiation aggregation properties of thecomposition A tend to increase, and resultingly, the light transmittinglayer is less likely to undergo cohesive failure, and the adhesion ofthe light transmitting layer tends to improve. Further, an effect oflimiting free movement of oxygen tends to increase, whereby the degreeof surface cure of the light transmitting layer tends to improve.

Further, the urethane oligomer may be produced by an optional knownmethod, and it is produced usually by oligomerizing a monomer having aurethane bond. The method of producing the monomer having a urethanebond is optional, and production may be carried out in accordance with aknown method such as a method of reacting a chloroformate with ammoniaor an amine, a method of reacting an isocyanate with a hydroxylgroup-containing compound, or a method of reacting urea with a hydroxylgroup-containing compound.

Further, in a case where such a monomer has a reactive group, themonomer is oligomerized to obtain a urethane oligomer. Usually, aurethane oligomer can be produced by an addition reaction of a compoundhaving two or more isocyanate groups in its molecule and a compoundcontaining a hydroxyl group by a conventional method.

The compound having two or more isocyanate groups in its molecule may,for example, be a polyisocyanate such as tetramethylene diisocyanate,hexamethylene diisocyanate, trimethylhexamethylene diisocyanate,bis(isocyanatomethyl)cyclohexane cyclohexane diisocyanate,bis(isocyanatocyclohexyl)methane, isophorone diisocyanate, tolylenediisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate,m-phenylene diisocyanate or naphthalene diisocyanate.

Among them, it is preferred to employ bis isocyanatomethyl)cyclohexane,cyclohexane diisocyanate, bis(isocyanatocyclohexyl)methane, isophoronediisocyanate or the like in view of favorable hue of the composition tobe obtained. Further, compounds having isocyanate groups may be usedalone or two or more of them may be used in an optional combination withan optional proportion.

Further, the compound containing a hydroxyl group is preferably a polyolcontaining two or more hydroxyl groups. Specifically, it may, forexample, be an alkyl polyol or a polyether polyol which is a multimerthereof, such as ethylene glycol, propylene glycol, butylene glycol,neopentyl glycol, trimethylolpropane, pentaerythritol, sorbitol,mannitol or glycerol, or a polyester polyol prepared from such a polyolor a polyhydric alcohol with a polybasic acid, or a polyester polyolsuch as polycaprolactone. These compounds containing a hydroxyl groupmay also be used alone or two or more of them may be used in an optionalcombination with an optional proportion.

Further, the urethane oligomer thus obtained is desirably one containingthe above polyether polyol as the compound containing a hydroxyl group.Specifically, the average content of the constituting units derived fromthe polyether polyol in one molecule of the urethane oligomer isdesirably usually at least 20 wt. %, preferably at least 25 wt. %, morepreferably at least 30 wt. %. Further, the upper limit is notparticularly limited, and is desirably usually at most 90 wt. %,preferably at most 80 wt. %, more preferably at most 70 wt.%.

If the content of the polyether polyol is too low, the lighttransmitting layer as a cured product tends to be fragile and theelastic coefficient of the light transmitting tends to be too high,whereby the internal stress is likely to occur and the lighttransmitting layer tends to deform. On the other hand, if the content istoo high the surface hardness of the light transmitting layer as a curedproduct tends to decrease, and such a problem tends to arise that thelight transmitting layer is easily scarred.

The addition reaction of the isocyanate compound with the hydroxylcompound may be carried out by an optional method. For example, amixture of the hydroxyl compound with an addition reaction catalyst,specifically dibutyltin laurate is dropwise added to the isocyanatecompound at a temperature of from 50° C. to 90° C.

Particularly in preparation of the urethane acrylate oligomer, it can beproduced by employing a compound having both hydroxyl group and(meth)acryloyl group as a part of the compound containing a hydroxylgroup. The amount of such a compound is optional, and it is usually from30 mol. % to 70 mol. % based on all the hydroxyl group-containingcompounds, and the molecular weight of the oligomer to be obtained canbe controlled depending upon its proportion.

Specifically, the compound having both hydroxyl groups and(meth)acryloyl group may, for example, be hydroxylethyl (meth acrylate,hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, an additionreaction product of a glycidyl ether compound with (meth)acrylic acid ora mono(meth)acrylate of a glycol compound.

Further, by an addition reaction of one molecule of the compound havingtwo or more isocyanate groups in its molecule with two molecules of thecompound having both hydroxyl group and (meth)acryloyl group, a urethaneoligomer having a (meth)acryloyl group on each terminal can be produced.

Particularly, the urethane oligomer having a (meth) acryloyl group oneach terminal has such an advantage that the adhesion and the degree ofsurface cure of the light transmitting layer to be obtained will furtherimprove.

Further, the urethane oligomer preferably further has an optional acidicgroup. By the urethane oligomer having an acidic group, such anadvantage can be obtained that the improved adhesion between theadherend and the light transmitting layer will be maintained with time(i.e. after time passes). The acidic group means a functional grouphaving acidity. The acidic group may, for example, be a sulfonic acidgroup, a phosphoric acid group, a carboxyl group or a neutral saltthereof with a tertiary amine compound or a metal salt thereof. Amongthem, a carboxyl group is most preferred. The urethane oligomer may haveone acidic group or two or more acidic groups in an optional combinationwith an optional proportion.

In order that the urethane oligomer has an acidic group, for example, asa raw material to be used for the production of the urethane oligomer,one having a carboxyl group may be employed Particularly, it ispreferred to employ a hydroxyl group-containing compound having acarboxyl group as the raw material compound.

The hydroxyl group-containing compound having a carboxyl group is notlimited and optional one may be employed. For example, a so-called aciddiol containing two or more hydroxyl groups may be preferably employed.Specifically, it may, for example, be a compound having two hydroxylgroups and two carboxyl groups in one molecule such as an alkanolcarboxylic acid or its caprolactone addition product such asdimethylolacetic acid, dimethylolpropionic acid, dimethylolbutanoicacid, dimethylolheptanoic acid, dimethylolnonanoic acid ordihydroxybenzoic acid; or a half ester compound of apolyoxypropylenetriol with maleic anhydride or phthalic anhydride.

Further, the molecular weight of the urethane oligomer is not limitedand is optional so long as the effects of the present invention are notsignificantly impaired. It is desirably usually at least 500, preferablyat least 1,000, more preferably at least 1,500, and usually at most10,000 preferably at most 8,000, more preferably at most 6,000. If themolecular weight is lower than the lower limit of this range, theshrinkage on curing tends to increase, and if it is higher than theupper limit, the viscosity may significantly increase, wherebyproducibility may deteriorate.

The urethane oligomers may be used alone or two or more of them may beused in an optional combination with an optional proportion.

(3-6) Other Components

In the composition A, another component may suitably be incorporated.

For example, a radiation-curable monomer and/or its oligomer(hereinafter suitably referred to as “radiation-curable component”) maybe incorporated. By employing this radiation-curable component, it ispossible to make the composition A be capable of being cured byradiation and to cure the composition A by irradiation with radiation,even when the urethane oligomer is not capable of being cured byirradiation.

Further, it is preferred to employ a bifunctional or trifunctional(meth)acrylate compound among radiation-curable components.

The bifunctional or trifunctional (meth)acrylate compound may beoptional, and it may, for example, be an aliphatic poly(meth)acrylate,an alicyclic poly(meth)acrylate or an aromatic poly(meth)acrylate.Specifically, it may, for example, be a bivalent (meth)acrylate such astriethylene glycol di-(meth)acrylate, hexanediol d (meth)acrylate,2,2-bis[4-(meth)aryloyloxyphenyl]propane,2,2-bis[4-(2-(meth)acryloyloxylethoxy)phenyl]propane,bis(hydroxymethyl)tricyclo[5.2.1.0^(2,6)]decane=dimethacrylate,p-bis[β-(meth)acryloyloxyethylthio]xylylene or4,4′-bis[β-(meth)acryloyloxyethylthio]diphenyl sulfone, a trivalent(meth)acrylate such as trimethylolpropane tris(meth)acrylate, glyceroltris(meth)acrylate or pentaerythritol tris(meth)acrylate, a tetravalent(meth)acrylate such as pentaerythritol tetrakis(meth)acrylate or anundefined multivalent (meth)acrylate such as epoxy acrylate. Among them,a bivalent (meth)acrylate is preferably employed in view ofcontrollability of the crosslink formation reaction.

Further, a trivalent or higher valent (meth)acrylate is preferablyemployed for the purpose of improving heat resistance and surfacehardness of the crosslinking structure of the light transmitting layer.Specifically, a trifunctional (meth)acrylate having an isocyanurateskeleton as well as the above-exemplified trimethylolpropanetris(meth)acrylate, etc. may, for example, be mentioned.

Further, a (meth)acrylate compound containing a hydroxyl group ispreferably employed for the purpose of improving the bonding propertiesand adhesion of the light transmitting layer Specifically, it may, forexample, be hydroxyethyl (meth)acrylate.

Further, among the above-exemplified (meth)acrylates, particularlypreferred is use of the following component I and the followingcomponent II with a view to realizing the transparency and a low degreeof optical distortion of the light transmitting layer in a well balancedmanner.

The component I is a bis(meth)acrylate having an alicyclic skeletonpresented by the following formula (1):

In the formula (1), each of R^(a) and R^(b) which are independent ofeach other, is a hydrogen atom or a methyl group, each of R^(c) andR^(d) which are independent of each other, is an alkylene group havingat most six carbon atoms, x is 1 or 2, and y is 0 or 1.

Specifically, the component I represented by the above formula (1) mayfor example, bebis(hydroxymethyl)tricyclo[5.2.1.0^(2,6)]decane=diacrylate,bis(hydroxymethyl)tricyclo[5.2.1.2^(2,6)]decane=dimethacrylate,bis(hydroxymethyl)tricyclo[5.2.1.0^(2,6)]decane-acrylate methacrylate ora mixture thereof,bis(hydroxymethyl)pentacyclo[6.5.1.1^(3,6)0^(2,7).0^(9,13)]pentadecane=diacrylate,bis(hydroxymethyl)pentacyclo[6.5.1.1^(3,6).0^(2,7).0^(9,13)]pentadecane=dimethacrylate,bis(hydroxymethyl)pentacyclo[6.5.1.1^(3,6).0^(2,7).0^(9,13)]pentadecane=acrylatemethacrylate or a mixture thereof. These components I may be used aloneor two or more of them may be used in an optional combination with anoptional proportion.

Further, the component II is a bis(meth)acrylate having a sulfur atomrepresented by the following formula (2):

In the above formula (2), R^(a) and R^(b) are the same as R^(a) andR^(b) in the above formula (1), respectively.

Each R^(e) independently is a C₁₋₆ alkylene group.

Each Ar independently is a C₆₋₃₀ arylene group or aralkylene group. Thehydrogen atoms in each Ar may be independently substituted by a halogenatom other than a fluorine atom.

Each X¹ independently is an oxygen atom or a sulfur atom.

In a case where all X¹'s are oxygen atoms, X² is a sulfur atom or asulfone group (—SO₂—) On the other hand, at least one of X¹'s is asulfur atom, X² is any one of a sulfur atom, a sulfone group, a carbonylgroup (—CO—), and a C₁₋₁₂ alkylene, aralkylene, alkylene ether,aralkylene ether, alkylene thioether and aralkylene thioether groups.

Each of j and p which are independent of each other, is an integer offrom 1 to 5, and k is an integer of from 0 to 10. In a case where k is0, X¹ is a sulfur atom.

Specifically, the component II represented by the above formula (2) may,for example, be α,α′-bis[β-(meth)acryloyloxyethylthio]-p-xylene,α,α′-bis[β-(meth)acryloyloxyethylthio]-m-xylene,α,α′-bis[β-(meth)acryloyloxyethylthio]-2,3,5,6-tetrachloro-p-xylene,4,4′-bis[β-(meth)acryloyloxyethoxy]diphenyl sulfide,4,4′-bis[β-(meth)acryloyloxyethoxy]diphenyl sulfone,4,4′-bis[β-(meth)acryloyloxyethylthio]diphenyl sulfide,4,4′-bis[β-(meth)acryloyloxyethylthio]diphenyl sulfone,4,4′-bis[β-(meth)acryloyloxyethylthio]diphenyl ketone,2,4′-bis[β-(meth)acryloyloxyethylthio]diphenyl ketone,5,5-tetrabromodiphenyl ketone,β,β′-bis[p-(meth)acryloyloxyphenylthio]diethyl ether orβ,β′-bis[p-(meth)acryloyloxyphenylthio]diethyl thioether. Suchcomponents II may be used alone or two or more of them may be used in anoptional combination with an optional proportion.

Among the above-described radiation-curable components,bis(hydroxymethyl)tricyclo[5.2.1.0^(2,6)]decane=dimethacrylate hasexcellent transparency and heat resistance and is particularlypreferably employed.

The radiation-curable components may be used alone or two or more ofthem may be used in an optional combination with an optional proportion.

The amount of the radiation-curable component to be used is optional solong as the effects of the present invention are not significantlyimpaired and it is desirably usually at most 50 wt. %, preferably atmost 30 wt. %, based on the composition excluding the above dispersiveinorganic components in the composition A, i.e. components other thanthe dispersed silica particles, the combined inorganic component and thesurface treating agent therefor.

Further, for example, a reactive diluent may be incorporated in thecomposition A for the purpose of adjusting the viscosity of thecomposition or for another purpose. The reactive diluent is a liquidcompound having a low viscosity and is usually a monofunctional lowmolecular weight compound. The reactive diluent is not limited and anoptional diluent may be employed so long as the effects of the presentinvention are not significantly impaired. It may, for example, be acompound having a vinyl group or a (meth)acryloyl group or a mercaptan.

However, the reactive diluent is preferably one capable of being curedby irradiation, and preferred is a compound having a vinyl group or a(meth)acryloyl group, for example. Specifically, such a compound may,for example, be an aromatic vinyl monomer, a vinyl ester monomer, avinyl ether, a (meth)acrylamide, a (meth)acrylate or a di(meth)acrylate,and preferred is a compound having a structure with no aromatic ring inview of the hue and the light beam transmittance. Among them,particularly preferred is a (meth)acrylate having an alicylic skeletonsuch as (meth)acryloylmorpholine, tetrahydrofurfuryl (meth)acrylate,benzyl (meth)acrylate, cyclohexyl (meth)acrylate or a (meth)acrylatehaving a tricyclodecane skeleton, a (meth)acrylamide such asN,N-dimethylacrylamide, or an aliphatic (meth)acrylate such ashexanediol di(meth)acrylate or neopentylglycol di(meth)acrylate in viewof favorable hue and viscosity.

Further, a compound having both hydroxyl group and (meth)acryloyl group,such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate orhydroxybutyl (meth)acrylate may also be used for the above purpose. Sucha compound may improve the adhesion of the light transmitting layer toan adherend in some cases.

The reactive diluents may be used alone or two or more of them may beused in an optional combination with an optional proportion.

Further, the amount of the reactive diluent to be used is optional solong as the effects of the present invention are not significantlyimpaired, and it is usually at least 0.5 wt. %, preferably at least 1wt. %, and usually at most 80 wt. %, preferably at most 50 wt. % basedon the composition except for the above dispersive inorganic componentsin the composition A, i.e. components excluding the dispersed silicaparticles, the combined inorganic component and the surface treatingagent therefor. If the amount is too small, the diluting effect may besmall, and if the amount is too large, the light transmitting layertends to be fragile and to have reduced mechanical strength, and theshrinkage on curing tends to be significant.

Further, it is usually preferred to incorporate a polymerizationinitiator in the composition A so as to initiate the polymerizationreaction which proceeds by active energy rays (such as ultravioletrays). As such a polymerization initiator, a radical generator which isa compound having characteristics to generate radicals by a light iscommonly used, and the polymerization initiator is not limited and knownone may optionally be employed.

The radical generator may, for example, be benzophenone, benzoin methylether, benzoin isopropyl ether, diethoxyacetophenone,1-hydroxycyclohexyl phenyl ketone, 2,6-dimethylbenzoyldiphenylphosphineoxide or 2,4,6-trimethylbenzoyldiphenylphosphine oxide. Among them,preferred is 1-hydroxycyclohexyl phenyl ketone,2,4,6-trimethylbenzoyldiphenylphosphine oxide or benzophenone.

The polymerization initiators may be used alone or two or more of themmay be used in an optional combination with an optional proportion.

The amount of the polymerization initiator to be used is optional solong as the composition A can be cured, and it is usually at least 0.001part by weight, preferably at least 0.001 part by weight, morepreferably at least 0.05 part by weight per 100 parts by weight of thetotal amount of the monomer containing a radiation-curable functionalgroup and/or its oligomer. Further, it is usually at most 10 parts byweight, preferably at most 8 parts by weight. If the amount is toolarge, not only the polymerization reaction will rapidly proceed and theoptical distortion of the light transmitting layer tends to increase,but also the hue of the light transmitting layer may deteriorate in somecases. Further, if the amount is too small, the composition A may notsufficiently be cured in some cases.

In a case where the polymerization reaction is initiated by electronbeams, although the above polymerization initiator may be employed it ispreferred not to use the polymerization initiator.

(3-7) Method of Forming Light Transmitting Layer

(3-7-1) Preparation of Composition A

The light transmitting layer is formed by curing the composition Acontaining at least the fine silica particles and the urethane oligomer.On that occasion, the composition A is prepared first, and the method ofpreparing the composition A is not limited, and an optional method maybe employed so long as the dispersive inorganic components can beuniformly dispersed in the above urethane oligomer. As specific examplesof such a method, the following methods may be mentioned.

(1) A method of preparing a powder of the fine silica particles,applying a proper surface treatment, and then directly dispersing thepowder in the urethane oligomer properly formed into a liquid state.

(2) A method of preparing the fine silica particles in the urethaneoligomer properly formed into a liquid state.

(3) A method of preparing the fine silica particles in a liquid medium,dissolving the urethane oligomer in the liquid medium and then removingthe solvent in the liquid medium.

(4) A method of removing the urethane oligomer in a liquid medium,preparing the fine silica particles in the liquid medium and thenremoving the solvent in the liquid medium.

(5) A method of preparing the fine silica particles and the urethaneoligomer in a liquid medium, and then removing the solvent in the liquidmedium.

Among the above methods for preparing the composition A, the method (3)is most preferred, by which a composition having high transparency andfavorable storage stability is likely to be obtained.

In the above method (3), specifically it is preferred to sequentiallycarry out (a) a step of hydrolyzing an oligomer of an alkoxysilane in aliquid medium such as a solvent, a surface treating agent or a diluentto prepare the fine silica particles, (b) a step of applying a surfacetreatment to the fine silica particles (c) a step of mixing urethaneoligomer with the fine silica particles, and (d) a step of removing thesolvent. According to this production method, a radiation-curable resincomposition having fine silica particles with a uniform particle sizehighly dispersed therein can more easily be obtained as the compositionA.

Now, steps in the method (3) will be explained in further detail below.

In the step (a), an oligomer of the alkoxysilane, a catalyst and waterare made to coexist in a liquid medium to carry out hydrolysis of theoligomer of an alkoxysilane to prepare the fine silica particles.

The liquid medium is not particularly limited, and preferred is onehaving compatibility with the urethane oligomer Specifically, theabove-described solvent, surface treating agent, diluent or the like maybe employed. Along them, in the method (3), the solvent is preferably analcohol or a ketone, particularly preferably a C₁₋₄ alcohol, acetone,methyl ethyl ketone or ethyl isobutyl ketone. Further, in the method(3), the amount of the liquid medium is preferably from 0.3 to 10 timesto the oligomer of an alkoxysilane.

Further, as the catalyst, the above described catalyst may be employed.Particularly in the method (3), a hydrolytic catalyst such as an organicacid such as formic acid or maleic acid; an inorganic acid such ashydrochloric acid, nitric acid or sulfuric acid; or a metal complexcompound such as aluminum acetylacetonate, dibutyltin dilaurate ordibutyltin dioctanoate is usually used. The amount of the catalyst to beused is preferably from 0.1 to 3 wt. % based on the oligomer of analkoxysilane.

Further, in the method (3), water is used usually in an amount of from10 wt. % to 50 wt % based on the oligomer of an alkoxysilane.

Then, in the step (b), a surface treatment is applied to the fine silicaparticles. The specific method of the surface treatment is optional, andusually it can be carried out by employing a surface treating agent inthe same manner as the above-described surface treatment on thedispersive inorganic components. Accordingly, the surface treating agentmay, for example, be a surfactant, a dispersing agent or a silanecoupling agent.

Then, in the step (c), the urethane oligomer and the fine silicaparticles are mixed. As mentioned above, in a case where the surfacetreatment is carried out employing a silane coupling agent in the step(b), the step (c) is carried out preferably after sufficient completionof the reaction in the step (b). Whether the reaction in the step (b) issufficiently completed can be confirmed by measuring the amount of theremaining silane coupling agent in the reaction liquid. Usually, thesufficient completion of the reaction in the step (b) can be confirmedwhen the amount of the remaining silane coupling agent in the reactionliquid becomes 10% or less of the charged amount.

The step (c) may be carried out at room temperature (25° C.), and in acase where the urethane oligomer has a high viscosity or in a case wherethe melting point of the urethane oligomer is room temperature (25° C.)or higher the step (c) may be carried out with heating at from 30 to 90°C. The mixing time is usually preferably from 30 minutes to 5 hours.

Then, in the step (d), removal of mainly the solvent employed as theliquid medium and the solvent such as an alcohol formed by thehydrolysis of the alkoxysilane oligomer is carried out. The solvents arenot necessarily completely removed so long as they are removed within arequired range, and they are preferably removed to such an extent thatsubstantially no solvent is contained. “Substantially no solvent iscontained” means such a state that the content of so-called organicsolvents having volatility or having a low boiling point is very low,and means a solvent content in the composition A of usually at most 5wt. %, preferably at most 3 wt. %, more preferably at most 1 wt %,furthermore preferably at most 0.1 wt. %. Briefly it means such a statethat no odor of the organic solvents is confirmed.

Otherwise, it means such a state that after the composition A is spincoated in a thickness of 100±15 μm heated at 70° C. for one minute andthen irradiated with ultraviolet rays of 3 J/cm² or irradiated withelectron beams of 5 Mrad, or cured until an evaluation result ◯ isachieved by the following method of evaluating the degree of surfacecure, no bubbles nor white turbidity due to volatilization of thesolvents remaining in the light transmitting layer as a cured product,is observed.

Method of evaluating the degree of surface cure: the composition A isirradiated with ultraviolet rays in a specified amount, the sample isgently pinched by the index finger and the thumb of the right handwearing a rubber glove so that the thumb is on the coated surface side,and the thumb is released from the coated surface. An evaluation is madeon the basis of standards ◯: no trace of the thumb visually observed, Δ:the trace slightly observed, and x: the trace clearly observed.

Further, when the solvents are removed removal is carried out by dryingthe solvents at a temperature usually within a range of from 10° C. to75° C. If the temperature is lower than the lower limit of this range,the solvents may not sufficiently be removed. On the other hand, if itis higher than the upper limit, the composition A is likely to gelate.The temperature may be controlled stepwise.

The time for removal of the solvents is preferably from 1 to 12 hours.

As the pressure condition at the time of removal of the solvents,removal is carried out preferably under reduced pressure of at most 20kPa, more preferably at most 10 kPa. Further, removal is carried outpreferably under a pressure of at least 0.1 kPa. Further, the pressuremay be gradually reduced.

The above-explained preferred preparation method has such an advantagethat ultrafine particles having a smaller particle size can be dispersedwithout aggregation even in a large amount as compared with a method ofadding a filler (such as fine silica particles) or a surface treatingagent such as a silane coupling agent to a resin composition (such asthe urethane oligomer) to disperse the filler. Accordingly, thecomposition A which is a radiation-curable resin composition to beobtained has the fine silica particles in a sufficient amount toincrease the dimensional stability and the mechanical strength of theresin dispersed therein without the radiation transmittance beingimpaired. Further, the light transmitting layer which is a product curedby radiation to be obtained by curing it has transparency, high surfacehardness and a low degree of shrinkage on curing. Preferably, it furtherhas dimensional stability and adhesion, and more preferably it has ahigh degree of surface cure in addition.

(3-7-2) Application of Composition A

The composition A prepared as mentioned above is applied directly on therecording and retrieving layer or in a case where another layer isformed on the recording and retrieving layer, via said another layer,and then cured to form the light transmitting layer. The method ofapplying the composition A is not limited and coating may be carried outby an optional method so long as a layer of the composition A can beformed with a predetermined thickness in accordance with the aimedthickness of the light transmitting layer.

Specifically, the coating method may, for example, be spin coating, dipcoating, roll coating, bar coating, die coating or spray coating.

Further, the thickness of the layer of the composition A applied is notlimited, and it is desirably usually at least 10 μm, preferably at least50 μm, more preferably at least 80 μm, and usually at most 1 mm,preferably at most 0.5 mm more preferably at most 0.3 μm. The thicknessof the applied layer is usually substantially the same as the thicknessof the light transmitting layer after curing or thicker by about 2%considering the shrinkage on curing.

Further, coating may be carried out in one time or may be dividedlycarried out in two or more times, but coating in one time iseconomically advantageous and preferred.

(3-7-3) Curing of Composition A

The composition A applied in the form of a layer on the recording andretrieving layer is cured to form the light transmitting layer. Forcuring the composition A, so-called “radiation curing” to irradiate thecomposition A with radiation (active energy rays or electron beams) toinitiate the polymerization reaction of the monomer or the oligomer inthe composition A, is carried out.

Specific procedures and conditions for the radiation curing are optionalso long as the composition A can be cured by the radiation curing.Accordingly, the manner of the polymerization reaction at the time ofthe radiation curing is not limited, and an optional knownpolymerization manner such as radical polymerization, anionicpolymerization, cationic polymerization or coordination polymerizationmay be employed. Among these polymerization manners, most preferred isradical polymerization. The reason is not clearly understood, but isestimated to be due to homogeneity of the product by homogeneousprogress of the initiation of the polymerization reaction in thepolymerization system in a short time.

The radiation may be electromagnetic waves (such as gamma rays, X-rays,ultraviolet rays, visible light, infrared rays or microwaves) orparticle beams (such as electron beams, alpha rays, neutron beams oratomic beams) which have such a function that they act on thepolymerization initiator which initiates a required polymerizationreaction, to generate chemical species which initiates thepolymerization reaction.

As examples of the preferred radiation, ultraviolet rays, visible lightand electron beams are preferred, and ultraviolet rays and electronbeams are more preferred, in view of the energy and that a generalpurpose light source can be used.

In the case of employing ultraviolet rays, usually a method may beemployed wherein a photoradical generator which generates radicals byultraviolet rays is used as a polymerization initiator and ultravioletrays are used as the radiation. As the photoradical generator, a knowncompound called a photopolymerization initiator or a photoinitiator mayoptionally be used Specifically, the photoradical generator may, forexample, be benzophenone, 4,4-bis(diethylamino)benzophenenone,2,4,6-trimethylbenzophenone, methylorthobenzoyl benzoate,4-phenylbenzophenone, thioxanthone, diethylthioxanthone,isopropylthioxanthone, chlorothioxanthone, t butylanthraquinone2-ethylanthraquinone, diethoxyacetophenone,2-hydroxy-2-methyl-1-phenylpropane-1-one, benzyl dimethyl ketal,1-hydroxycyclohexyl phenyl ketone, benzoin methyl ether, benzoin ethylether, benzoin isopropyl ether, benzoin isobutyl ether,2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropan-1-one,2,4,6-trimethylbenzoyldiphenylphosphine oxide,bis(2,6-dimethoxylbenzoyl)-2,4,4-trimethylpentylphosphine oxide,bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide or methylbenzoylformate. Such photoradical generators may be used alone or two ormore of them may be used in an optional combination with an optionalproportion.

Further, in a case where the composition A is employed for e.g. anoptical recording medium employing a laser having a wavelength of from380 nm to 800 nm as a light source, it is preferred to suitably selectthe type and the amount of the photoradical generator so that a laserbeam required for reading can sufficiently be transmitted through thelight transmitting layer formed by curing the composition A. In such acase, it is particularly preferred to use a photoradical generatorsensitive to short wavelength, which provides a light transmitting layerwhich hardly absorbs a laser beam, as the photoradical generator.Specifically such a photoradical generator sensitive to short wavelengthmay, for example, be benzophenone, 2,4,6-trimethylbenzophenone,methylorthobenzoyl benzoate, 4-phenylbenzophenone, diethoxyacetophenone,2-hydroxy-2-methyl-1-phenylpropan-1-one, benzyl dimethyl ketal,1-hydroxycyclohexyl phenyl ketone, benzoin methyl ether, benzoin ethylether, benzoin isopropyl ether, benzoin isobutyl ether or methylbenzoylformate. Such photoradical generators may also be used alone ortwo or more of them may be used in an optional combination with anoptional proportion.

In the composition A, the content of the photoradical generator is notlimited and is optional so long as the effects of the present inventionare not significantly impaired. It is desirably usually at least 0.001part by weight, preferably at least 0.01 part by weight, more preferablyat least 0.1 part by weight, and usually at most 10 parts by weight,preferably at most 9 parts by weight, more preferably at most 7 parts byweight per 100 parts by weight of the total amount of componentsexcluding the dispersed silica particles, the combined inorganiccomponent and the surface treating agent therefor in the composition A.If the content of the photoradical generator is lower than the lowerlimit of the above range, the mechanical properties of the composition Atend to be insufficient and if it is more than the upper limit, theradiation curing properties of the film of the composition A tend to bepoor, or the light transmitting layer may be colored, whereby reading ofinformation utilizing light tends to be impaired.

On that occasion, a sensitizer may be used in combination as the caserequires. The sensitizer is not limited and a known sensitizer mayoptionally be employed Specifically, the sensitizer may, for example, bemethyl 4-dimethylaminobenzoate, ethyl 4-dimethylaminobenzoate, amyl4-dimethylaminobenzoate or 4-dimethylaminoacetophenone. Such sensitizersmay be used alone or two or more of them may be used in an optionalcombination with an optional proportion.

Further, as the ultraviolet rays, one having a wavelength within a rangeof usually from 200 to 400 nm, preferably from 250 to 400 nm, may beemployed. As an apparatus to apply ultraviolet rays, an optional knownapparatus such as a high-pressure mercury lamp, a metal halide lamp oran ultraviolet lamp having a structure to generate ultraviolet rays bymicrowaves may preferably be employed. Preferred is a high-pressuremercury lamp. The output of the apparatus is usually from 10 to 200W/cm, and the apparatus is preferably installed with a distance of from5 to 80 cm from the object to be irradiated, whereby the degree ofphoto-deterioration, thermal deterioration, thermal deformation and thelike of the irradiated object tends to be low.

Further, the composition A can be preferably cured also by electronbeams, whereby a cured product excellent in mechanical propertiesparticularly tensile elongation properties will be obtained. In the caseof employing electron beams, although the light source and irradiationapparatus therefor tend to be expensive their use is preferred in somecases, since it is possible to omit use of the initiator, and nopolymerization inhibition by oxygen will occur, whereby favorable degreeof surface cure will be obtained. The manner of an electron beamirradiation apparatus to be used for irradiation with electron beams isnot limited and an optional apparatus may be employed, and a curtaintype, an area beam type, a broad beam type or a pulse beam type may, forexample, be mentioned. Further, the accelerating voltage at the time ofirradiation with electron beams is usually preferably from 10 to 1,000kV.

Further, the intensity of the radiation applied at the time of curing byradiation is optional so long as the composition A can be cured, and itis desired to apply the radiation with an energy of usually at least 0.1J/cm², preferably at least 0.2 J/cm². Further, it is desired to applythe radiation with an energy within a range of usually at most 20 J/cm²,preferably at most 10 J/cm², more preferably at most 5 J/cm²,furthermore preferably at most 3 J/cm²/particularly preferably at most 2J/cm². The intensity of the radiation can be optionally selected withinthis range depending upon the type of the composition A. For example, ina case where the composition A is a radiation-curable resin compositioncontaining a monomer having a urethane bond or a hydroxyalkylene groupand/or its oligomer including a urethane oligomer, the intensity of theradiation applied is preferably at most 2 J/cm². If the energy of theradiation applied is extremely low or the application time is extremelyshort, the polymerization tends to be incomplete, whereby no sufficientheat resistance nor mechanical properties of the light transmittinglayer may be obtained.

Further, the radiation application time is also optional so long as thecomposition A can be cured and it is usually at least one second,preferably at least 10 seconds. However, if the time is extremely inexcess deterioration represented by deterioration of the hue by lightsuch as yellowing may occur in some cases. Accordingly, the applicationtime is usually at most 3 hours and it is preferably at most about 1hour in view of acceleration of the reaction and productivity.

Further, the application of radiation may be carried out in one step ormay be dividedly carried out in two or more steps. As the source ofradiation, usually a diffusion source of radiation from which theradiation radiates in all directions is employed.

(3-8 Physical Properties of Light Transmitting Layer

It is usually preferred that the above formed light transmitting layeris insoluble and infusible in a solvent or the like, it has propertiesadvantageous for an application to an optical component even if it isformed into a thick film, and it is excellent in adhesion and degree ofsurface cure. Specifically, it preferably has a low degree of opticaldistortion (low birefringence), a high light beam transmittance,dimensional stability, high adhesion, a high degree of surface cure andheat resistance at a certain level or more. Further, it preferably has adegree of shrinkage on curing as low as possible.

The physical properties will be explained in further detail below.

The thickness of the light transmitting layer is not limited and canoptionally be set, and it is usually at most 5 mm, preferably at most 2mm, more preferably at most 1 mm, furthermore preferably at most 500 μm,and it is usually at least 0.1 μm, preferably at least 1 μm, morepreferably at least 10 μm, furthermore preferably at least 50 μmparticularly preferably at least 70 μm most preferably at least 90 μm.

The transparency of the light transmitting layer is optional within arange where it is not against the scope of the present invention, andthe light beam transmittance per 0.1 mm of the light path length at 550nm is desirably usually at least 80%, preferably at least 85%, morepreferably at least 89%. More preferably, the light beam transmittanceper 0.1 mm of the light path length at 400 nm is usually at least 80%,preferably at least 85%, more preferably at least 89%. Particularlypreferred is one having the above light beam transmittance per 1 mm ofthe light path length. The upper limit of the light beam transmittanceis ideally 100%. The light beam transmittance may be measured, forexample, by means of an ultraviolet/visible spectrophotometer modelHP8453 manufactured by Hewlett-Packard Development Company, L.P at roomtemperature.

Further, the surface hardness of the light transmitting layer isoptional, and the surface hardness by a pencil hardness test inaccordance with JIS K5400 is preferably at least 2B Further, it ispreferably at least HB, more preferably at least F, furthermorepreferably at least H. Further, it is preferably at most 7H. In such acase, the light transmitting layer preferably satisfies the abovehardness even if it is a cured product cured on an inorganic substrateof e.g. glass or a metal or a resin substrate. More preferably, itsatisfies the above hardness even when it is a cured product cured on aplastic substrate of e.g. a polycarbonate. If the hardness is too low,the surface is likely to be scarred. A too large hardness itself is notproblematic, but the light transmitting layer tends to be fragile, andcracks and pealing are likely to occur.

Further, the degree of shrinkage on curing when the composition A iscured to form the light transmitting layer is preferably as low aspossible, and it is usually at most 3 vol. %, preferably at most 2 vol.%. Measurement of the shrinkage on curing is usually substituted by amethod of coating a substrate with the composition A and measuring theamount of concave warp formed after the curing. As a specific measuringmethod, a film of the composition A with a thickness of 100±15 μm isformed on a circular polycarbonate plate having a diameter of 130 mm anda thickness of 1.2±0.2 mm by a spin coater, followed by irradiation withradiation in a specific amount, and then the polycarbonate plate is leftat rest on a platen for one hour. Then, the concave warp (spring) of thepolycarbonate plate caused by shrinkage on curing of the composition Ais measured. The concave warp is preferably at most 1 mm, morepreferably at most 0.5 mm, furthermore preferably at most 0.1 mm. Thelower limit of the concave warpage is ideally 0.1 mm. The “concave warp”means the amount of warp from the platen as observed when thepolycarbonate plate warps along with the shrinkage on curing of thecomposition A formed on the polycarbonate plate. To determine the“concave warp”, it is possible to measure the amounts of warp at pluralpoints on the polycarbonate plate to obtain an average of the measuredvalues.

The degree of thermal expansion of the light transmitting layer isoptional, and a smaller thermal expansion means more favorabledimensional stability and is preferred. For example, of the lighttransmitting layer, the coefficient of linear expansion which is one ofspecific indices of thermal expansion is as small as possible and it isdesirably usually at most 13×10⁻⁵/° C., preferably at most 12×10⁻⁵/° C.more preferably at most 10×10⁻⁵/° C., furthermore preferably at most8×10⁻⁵/° C. The lower limit of the coefficient of thermal expansion ispractically at a level of 2×10⁻⁵/° C. The coefficient of linearexpansion may be determined for example, in such a manner that employinga plate test specimen of 5 mm×5 mm×1 mm, by a compression methodthermomechanical analyzer (TMA, model SSC/5200 manufactured by SeikoInstruments Inc.) with a load of 1 g at a temperature-increasing rate of10° C./min, the coefficient of linear expansion is evaluated every 10°C. within a range of from 40° C. to 100° C., and the average is obtainedas a representative value.

In addition, the adhesion of the light transmitting layer is preferablyhigh. As a method of measuring the adhesion, for example, thecomposition A in an amount with which a coating film can be formed isdropped on a 10 cm square optically polished glass plate, followed byirradiation with radiation in a specific amount, and then the glassplate is left to stand at room temperature for one hour. Then, thecenter portion of the cured composition A is notched by a cutter knifeso that the notch reaches the glass surface, and the glass plate isfurther left to stand at room temperature for 14 days, and an evaluationis made by whether separation at the interface between the cured productof the composition A (corresponding to the light transmitting layer) atthe notched portion and the glass surface is visually observed or not.Using five samples, and an evaluation is made based on standards ⊚: noseparation observed in all samples, ◯: no separation observed in two ormore samples, Δ: no separation observed in only one sample, and x:separation visually observed in all samples. As the evaluation of theadhesion, preferred is ◯ or ⊚, more preferably ⊚. Further, the lighttransmitting layer more preferably has the above adhesion on a plasticsubstrate of e.g. a polycarbonate, rather than an optically polishedglass plate.

In addition, the degree of surface cure of the light transmitting layeris preferably hard. The degree of surface cure is determined as follows.After irradiation with ultraviolet rays in a specific amount a sample isgently pinched with the index finger and the thumb of the right handwearing a rubber glove so that the thumb is on the coated surface side,and the thumb is released from the coated surface. An evaluation is madeon the basis of standards ◯: no trace of the thumb visually observed, Δ:the trace slightly observed, and x: the trace clearly observed. As theevaluation of the surface hardness, referred is ◯.

Further, with respect to the heat resistance of the light transmittinglayer, the glass transition temperature as measured by differentialscanning calorimetry (DSC), thermomechanical analysis (TMA) or dynamicviscoelasticity measurement is desirably usually at least 120° C.,preferably at least 150° C., more preferably at least 170° C. However,the glass transition point is practically at most 200° C.

Further, the light transmitting layer is preferably insoluble in varioussolvents Typically, it is preferably insoluble in a solvent such astoluene, chloroform, acetone or tetrahydrofuran.

(4) Stainproof Layer

The stainproof layer is a layer to be formed on the light transmittinglayer so as to prevent the optical recording medium from being stained.Further, the stainproof layer preferably has stainproof properties andlubricity i.e. water repellency and oil repellency. Therefore, in theoptical recording medium of the present invention, the stainproof layeris formed to contain the component B. The component B is an alkoxysilanecompound containing a fluorine atom (hereinafter suitably referred to as“fluorine-containing alkoxysilane compound”) and/or a hydrolysate of thefluorine-containing alkoxysilane compound.

(4-1) Component B

As mentioned above, the component B is a fluorine-containingalkoxysilane compound and/or a hydrolysate of the fluorine-containingalkoxysilane compound.

The fluorine-containing alkoxysilane compound is not particularlylimited and optional known one may be employed. It may, for example, bea silane coupling agent containing a fluoroalkyl group or a fluoroarylgroup.

Among silane coupling agents having a fluoroalkyl group, preferred is aC₃₋₁₂ alkoxysilane having a fluoroalkyl group. Specifically, it may, forexample, be (3,3,3-trifluoropropyl)triethoxysilane,(tridecafluoro-1,1,2,2-tetrahydrooctyl)triethoxysilane,(peptadecafluoro-1,1,2,2-tetrahydrodecyl)triethoxysilane,(henicosafluoro-1,1,2,2-tetrahydrodecyl triethoxysilane, or a multimerof such a compound, or a modified product obtained by condensing such acompound with another alkoxysilane compound and/or a hydroxylgroup-containing compound.

Further, among silane coupling agents having a fluoroaryl group,preferred is a C₆₋₉ alkoxysilane having a fluoroaryl group.Specifically, it may, for example, be pentafluorophenyltriethoxysilane,(pentafluorophenyl)propyltriethoxysilane, a multimer of such a compoundor a modified product obtained by condensing such a compound withanother alkoxysilane compound and/or a hydroxyl group-containingcompound.

As commercial products, OPTOOL DSX (manufactured by DAIKIN INDUSTRIES,Ltd.), FluoroSurf series FS-1000, series FS-2000, series FG-3000, seriesFG-4000 and series FG-5000 (manufactured by Fluoro Technology) and NovecEGC-1720 (manufactured by Sumitomo 3M Limited) may, for examples bementioned. Among them, OPTOOL DSX, FluoroSurf FG-5010 among seriesFG-5000 and Novec EGC-1720 are preferred, and Novec EGC-1720 is mostpreferred.

Such fluorine-containing a alkoxysilane compounds may be used alone ortwo or more of them may be used in an optional combination with anoptional proportion.

Further, as the component B, a hydrolysate of the fluorine-containingalkoxysilane compound may be used instead of the fluorine-containingalkoxysilane compound or together with the fluorine-containingalkoxysilane compound. Usually, a hydroxyl group forms from thefluorine-containing alkoxysilane compound by a hydrolytic reaction withwater. This hydroxyl group has high reactivity in many cases, andaccordingly it is possible to increase the adhesion of the stainprooflayer employing the above hydrolysate to the light transmitting layer.

Water required for the hydrolytic reaction of the fluorine-containingalkoxysilane compound is not limited, and it may be usual running water,pure water or ion exchanged water, or may be water mixed with a watercomposition such as a hydrated compound or may be water in the air. In acase where the hydrolysis is carried out employing water. In the air,the hydrolytic reaction will moderately proceed.

Further, for the hydrolysis for the fluorine-containing alkoxysilanecompound, a catalyst may be employed. The catalyst to be employed forthe hydrolysis is not limited and optional one may be employed. It may,for example, be a catalytic component such as a metal chelate compound,an organic acid, a metal alkoxide or a boron compound. Such catalyticcomponents may be used alone or two or more of them may be used in anoptional combination with an optional proportion. Further, such acatalytic component may be made to coexist with the fluorine-containingalkoxysilane compound immediately before the hydrolysis, or may be madeto preliminarily coexist with the fluorine-containing alkoxysilanecompound together with a small amount of water so that a desiredhydrolytic reaction will proceed.

The amount of such a catalytic component to be used is not particularlylimited and is optional within a range where its effect can sufficientlybe obtained. It is desired to employ the catalytic component in anamount of usually at least 0.1 part by weight, preferably at least 0.5part by weight per 100 parts by weight of the alkoxysilane oligomer.However, the effect will no longer improve even when a too large amountis used, and thus its amount is desirably usually at most 10 parts byweight, preferably at most 5 parts by weight.

Further, in the stainproof layer, an optional additive may beincorporated in addition to the above component B and the solvent. Theadditive to be used together with the component B is not limited and isoptional within a range where the effects of the present invention willnot significantly be impaired. For example, another compound containinga fluorine atom or a silicone compound may be employed.

Before the stainproof layer is formed, the component B and the additivesuitably used as the case requires are in a state of a compositiondissolved or dispersed in the solvent (hereinafter suitably referred toas “coating composition”), because usually a coating method is employedfor formation of the stainproof layer.

The solvent to be used for the coating composition is not limited and isoptional so long as the effects of the present invention are notsignificantly impaired. Usually a halogen organic solvent is preferred,and a fluorine solvent is particularly preferred. Specific preferredcommercially available solvents include Fluorinert FC-87, FluorinertFC-72, Fluorinert FC-84 Fluorinert FC-77, Fluorinert FC-3283, FluorinertFC-40, Fluorinert FC-43, Fluorinert FC-70, HFE-7100 and HFE-7200(manufactured by Sumitomo 3M Limited/ASAHIKLIN AK-225, ASAHIKLINAK-225AES, ASAHIKLIN AK-3000 ASAHIKLIN AE-3100, Clin Dry and Clin Dry α(manufactured by Asahi Glass Company, Limited. Among them, FluorinateFC-3283 and Fluorinate FC-40 are most preferred in view of appropriateevaporation rate with a boiling point within a range of from 80 to 170°C. and favorable coating spreadability.

Further, the compositions of the component B and the coating compositionare also optional within a range where the effects of the presentinvention are not significantly impaired.

However, the proportion by weight of the solid component in the coatingcomposition i.e. the fluorine-containing alkoxysilane compound and/orits hydrolysate and a solid additive to be suitably employed, is usuallyat least 0.01 wt. %, preferably at least 0.05 wt. %, more preferably atleast 0.07 wt. % and usually at most 1 wt. %, preferably at most 0.5 wt.%, more preferably 0.2 wt. %. If the proportion is lower than the lowerlimit of this range, the stainproof layer may be separated, whereby thestainproof properties significantly decrease, and if it is above theupper range, the coating properties may significantly decrease, or theeconomical efficiency may decrease since a higher effect will no longerbe obtained even at a proper concentration or above.

(4-2) Formation of Stainproof Layer

The method for forming the stainproof layer is not limited, and anoptional method may be employed so long as a layer containing thefluorine-containing alkoxysilane compound and/or its hydrolysate can beformed. For example, a stainproof layer containing the component B canbe formed by preparing a coating composition containing the component Band the solvent and the additive to be suitably employed, and applyingthe coating composition to the above-described light transmitting layerdirectly or in a case where another layer is formed on the lighttransmitting layer, via said another layer, and drying the solvent.

When the coating composition is applied, the coating method is notlimited, and an optional known method such as spin coating, dip coatingor spray coating may be employed.

Further, the drying method is also optional. For example, drying byheating at a temperature of from 30° C. to 1000° may be carried out, ordrying may be carried out at room temperature. Further, for example, thedrying time is preferably one day, more preferably three days.Sufficient drying makes it possible to improve stainproof properties andadhesion to the light transmitting layer.

(4-3) Physical Properties of Stainproof Layer

The stainproof layer formed to contain the component B is excellent instainproof properties and adhesion and it can thereby prevent theoptical recording medium of the present invention from being stained.Further, when the stainproof layer is formed on the above lighttransmitting layer, the adhesion of the stainproof layer to the lighttransmitting layer can be increased very high.

The stainproof properties can be quantitatively evaluated by means ofpure water contact angle or hexadecane contact angle.

The pure-water contact angle of the stainproof layer is usually at least85°, preferably at least 95°, more preferably at least 100°. On theother hand, the upper limit of the pure water contact angle ispractically 150°.

Further, the hexadecane contact angle of the no stainproof layer isusually at least 40°, preferably at least 50°, more preferably at least60°. On the other hand, the upper limit of the hexadecane contact angleis practically 150°.

Further as one index of the stainproof properties, it is preferred thata fingerprint hardly attaches to the above stainproof layer and thefingerprint is invisible. Quantitatively, such an evaluation can be madethat a fingerprint hardly attaches and is invisible, for example, whenthe sebum on the nose is taken on the thumb, the thumb is pressed on thesurface of the stainproof layer, and the fingerprint is observed by anoptical microscope, whereupon the grease spot to be observed ispreferably at most 30 μm, more preferably at most 10 μm. It is idealthat the grease spot is 0 μm, that is, no grease spot is observed.

On the other hand, the adhesion of the stainproof layer can be evaluatedby examination of repellency using a felt-tip pen. Specifically, anevaluation can be made by means of a felt-tip pen writing test. Forexample, writing is carried out on the surface of the stainproof layeremploying a felt-tip pen (PIN-03A manufactured by MITSUBISHI PENCIL COMPNY LIMITED) with a usual strength (0.05 MPa to 0.1 MPa), and thestainproof layer can be considered to have favorable adhesion when itrepels the ink of the felt-tip pen. If it has poor adhesion, whenwriting by a felt-tip pen is carried out, the stainproof layer will peeloff due to the shear stress, and the ink of the felt-tip pen will not berepelled.

Further, the thickness of the stainproof layer is not limited and isoptional within a range where the effects of the present invention arenot significantly impaired. It is usually at least 5 nm, preferably atleast 10 nm, and usually at most 1,000 nm, preferably at most 100 nm,more preferably at most 50 nm. If the thickness is lower than the lowerlimit of this range, uneven stainproof properties may be obtained, andif it is above the upper limit, the stainproof layer tends to peel off.

Further, the stainproof layer preferably has a high transmittance of alight to be employed for recording and retrieving on the opticalrecording medium of the present invention. For example, thetransmittance of a light having a wavelength of 550 nm is desirablyusually at least 85%, preferably at least 89%. The upper limit of thetransmittance is ideally 100%.

Further, the stainproof layer usually contains a solvent employed at thetime of formation of the stainproof layer. Usually, the solventcontained in the coating composition cannot completely be removed evenby drying and remains in the stainproof layer. Accordingly, for example,when a halogen organic solvent is employed as the solvent of the coatingcomposition, the stainproof layer will also contain the halogen organicsolvent.

However, the proportion of the organic solvent in the stainproof layeris usually preferably as small as possible, and specifically, theproportion by weight of the solvent in the stainproof layer is desirablyusually at most 30 wt. %, preferably at most 10 wt. %, more preferablyat most 5 wt. %. The lower limit of the proportion of the organicsolvent is practically 100 ppm. ppm represents a proportion based on theweight.

(5) Other Layers

The optical recording medium of the present invention may further haveanother layer. The position of such a layer provided is optional, and itcan be formed at a proper position depending upon the type, the purposeof use and the application of the optical recording medium.

However, the above stainproof layer is preferably an outermost layer ofthe optical recording medium, so as to securely prevent stains.

Further, the light transmitting layer is preferably formed directly onthe recording and retrieving layer, and the stainproof layer ispreferably formed directly on the light transmitting layer, so as toincrease the adhesion between the respective layers.

The total thickness of the light transmitting layer and the stainprooflayer is considered to be at a level of 100 μm in the case of aso-called blu-ray disc. It is preferred to adjust the thickness to be100 μm with a margin of 10 μm considering the refractive indices of suchlayers. Further, the thickness of the light transmitting layer ispreferably at least 80%, more preferably at least 90% of the totalthickness. On the other hands the thickness of the stainproof layer ispreferably at least 0.1%, more preferably at least 1% of the totalthickness. The thickness of the stainproof layer is usually at most 20%preferably less than 20%, more preferably at most 10%, particularlypreferably less than 10% of the total thickness.

(6) Effects and the Like

The optical recording medium of the present invention constituted asmentioned above has sufficient stainproof properties, since on anoptical recording medium having a substrate and a recording andretrieving layers a light transmitting layer formed by curing acomponent A i.e. a composition A containing silica particles and anoligomer having a urethane bond and capable of being cured byirradiation with radiation, and a stainproof layer containing acomponent B i.e. an alkoxysilane compound containing a fluorine atomand/or a hydrolysate of the alkoxysilane compound. Further, since thelight transmitting layer can be produced by applying the composition Aand curing it by irradiation with radiation, production of the opticalrecording medium is easily carried out, and the adhesion of the lighttransmitting layer to e.g. the recording and retrieving layer can beincreased.

Now, advantages of the present invention in comparison with prior artwill be explained in detail below.

For example, a conventional protective layer as proposed in PatentDocument 2 has no sufficient stainproof properties. Further, even when afluorine compound is added to the composition as disclosed in PatentDocument 2, no sufficient stainproof properties can be imparted to theoptical recording medium. However, the optical recording medium of thepresent invention has sufficient stainproof properties.

Further, in technique as disclosed in Patent Document 1, the layerformed on the surface of the optical recording medium has a three-layerstructure, and such leads to a high cost, poor producibility and thuspoor practicability. However, the optical recording medium of thepresent invention can be produced by a simple and industriallyadvantageous process since functions such as stainproof properties,achieved by the three-layer structure in Patent Document 1, can beachieved by two layers of the light transmitting layer and thestainproof layer.

Further, in the optical recording medium of the present invention, thelight transmitting layer is formed by curing by radiation employing aurethane oligomer, whereby adhesion between the light transmitting layerand the recording and retrieving layer or the like can be improved.

Further, in a case where dispersive inorganic components are notdispersed in the light transmitting layer (top layer in PatentDocument 1) as in a conventional case, the adhesion between the lighttransmitting layer and the stainproof layer containing an inorganiccomponent may be low, but in a case where the stainproof layer of theoptical recording medium of the present invention is directly formed onthe surface of the light transmitting layer, the adhesion between thestainproof layer and the light transmitting layer can be improved.

Further, cracks or warp occurs if the light transmitting layer (such asthe anchor layer in Patent Document 1) is made thick as in aconventional case, but in the optical recording medium of the presentinvention, the light transmitting layer is formed by using a urethaneoligomer and dispersive inorganic components in combination whereby itis possible to suppress occurrence of cracks or warp as observed inconventional products.

EXAMPLES

Now, the present invention will be explained in further detail withreference to Examples. However, the present invention is not limited tothe following Examples, and optional modifications are possible within arange not to depart from the scope of the present invention.

Evaluation Method

Test on Stainproof Properties

Evaluated based on the pure water contact angle and the hexadecanecontact angle employing a contact angle meter model CA-DT, manufacturedby Kyowa Interface Science Co., Ltd.).

Test on Adhesion

Employing a felt-tip pen (PIN-03A manufactured by MITSUBISHI PENCILCOMPANY LIMITED), a straight line with a length of 2 cm was drawn on thesurface of the stainproof layer with a strength of 0.05 MPa freehandEvaluation was made on the basis of standards ◯: the ink of the felt-tippen repelled, and X: the ink of the felt-tip pen not repelled

Preparation

(a) Preparation of Tetramethoxysilane Oligomer

1170 g of tetramethoxysilane as an alkoxysilane and 370 g of methanolwere mixed, and 111 g of 0.05% hydrochloric acid was added to carry outa hydrolytic reaction at 65° C. for 2 hours.

Then, the temperature in the system was increased to 130° C., and theformed methanol was removed, and then the temperature was graduallyincreased to 150° C. while blowing a nitrogen gas, and such a state waskept for 3 hours to remove the tetramethoxysilane monomer thereby toobtain an oligomer of tetramethoxysilane.

(b) Preparation of Silica Sol

To 122.7 g of the oligomer of tetramethoxysilane obtained in the abovestep (a), 225.3 g of methanol was added, followed by uniform stirring,and then 24.6 g of a 5% methanol solution of aluminum acetylacetonatewas added, followed by stirring for 30 minutes. To this solution, 26.0 qof demineralized water was gradually dropwise added with stirring, andstirring was continued as it was at 60° C. for 2 hours to let finesilica particles grow.

Then, 119.6 g of acryloxypropyltrimethoxysilane as a silane couplingagent (surface treating agent) and 4.0 parts of maleic acid were added,followed by aging with stirring at 60° C. for 2 hours. Then, 53.5 g ofdemineralized water and 119.6 g of acryloxypropyltrimethoxysilane weregradually added, followed by stirring at 60° C. for 4 hours so that thesilane coupling agent was reacted to the surface of the fine silicaparticles to carry out a surface treatment, thereby to prepare a silicasol.

(c) Synthesis of Urethane Acrylate Oligomer

Into a 2 L four-necked flask, 222.3 g of isophorone diisocyanate and0.06 g of dibutyltin laurate were put, heated to from 70 to 80° C. in anoil bath and calmly stirred until the temperature became constant. Afterthe temperature became constant, a mixture of 29.6 g ofdimethylolbutanoic acid and 255.0 g of polytetramethylene glycol wasdropwise added from a dropping funnel, followed by stirring for 2 hourswhile keeping the temperature at 80° C. After the temperature wasdecreased to 70° C., a mixture of 145.0 g of hydroxyethyl acrylate and0.3 g of methoquinone was dropwise added from a dropping funnel, andafter completion of the dropwise addition, stirring was carried out for10 hours while keeping the temperature at 80° C. to synthesize aurethane acrylate oligomer as a urethane oligomer. Immediately after thesynthesis, 217.4 g of isobornyl acrylate was added followed by stirringto prepare a urethane resin composition.

(d) Preparation of Silica-Containing Composition (Component A) for LightTransmitting Layer

Into a 500 cc round bottom flask, 79.7 g of the silica sol prepared inthe step (b), 53.2 g of the urethane resin composition prepared in thestep (c), and as radiation-curable components, 10.6 g (10 wt. % of thetotal resin components) of polypropylene glycol diacrylate (APG 400,manufactured by SHIN-NAKAMURA CHEMICAL CO., LTD, molecular weight about500), 31.9 g of isobornyl acrylate and 10.6 g of hydroxyethyl acrylate,and as photoradical generators, 2.5 g of 1-hydroxycyclohexyl phenylketone and 2.5 g of benzophenone were added, followed by stirring atroom temperature for 2 hours to obtain a transparent radiation-curableresin composition. Further, this radiation-curable resin composition wassubjected to evaporation under reduced pressure at 50° C. for 1 hour toremove low-boiling components contained in the radiation-curable resincomposition thereby to prepare a silica-containing composition for lighttransmitting layer as the component A.

(e) Preparation of Composition Containing no Silica for Formation ofLight Transmitting Layer

53.2 g of the urethane resin composition prepared in the above step (c),as radiation-curable components, 10.6 g (10 wt. % of the total resincomponents) of polypropylene glycol diacrylate (APG400, manufactured bySHIN-NAKAMURA CHEMICAL CO., LTD, molecular weight: about 500), 31.9 g ofisobornyl acrylate and 10.6 g of hydroxyethyl acrylate, and asphotoradical generators, 2.5 g of 1-hydroxycyclohexyl phenyl ketone and2.5 g of benzophenone were added, followed by stirring at roomtemperature for 2 hours to prepare a composition containing no silicafor light transmitting layer.

(f) Preparation of Composition α for Stainproof Layer

Novec EGC-1720 (manufactured by Sumitomo 3M Limited, solid componentconcentration: 0.1 wt. %) was employed as a composition α for stainprooflayer. The solid component in Novec EGC-1720 corresponds to thecomponent B.

(g) Preparation of Composition β for Stainproof Layer

Into a flask, 0.5 g of OPTOOL DSX (manufactured by DAIKIN INDUSTRIES,Ltd., solid component concentration 20 wt. %) and 10 g of F-3283(manufactured by Sumitomo 3M Limited) as a solvent were weighted,followed by stirring by a magnetic stirrer at room temperature for 30minutes to prepare a composition β (solid component 0.1 wt. %) forstainproof layer. The solid component in OPTOOL DSX corresponds to thecomponent B.

Example 1

To a circular polycarbonate substrate having a diameter of 130 mm and athickness of 1.2 mm, the silica-containing composition (component A) forlight transmitting layer prepared in the step (d) was applied in athickness of 100±15 μm by a spin coater, and irradiated with ultravioletrays (irradiation intensity 1 J/cm²) for 15 seconds by a high-pressuremercury lamp with an output of 80 W/cm located at a position with adistance of 15 cm from the film of the composition to cure thecomposition thereby to form a light transmitting layer.

The substrate was left to stand at room temperature for one hour, andthen the composition α for stainproof layer (coating compositioncontaining the component B) prepared in the step (f) was applied to thesubstrate by a spin coater at 1,000 revolutions for 10 seconds, and thesubstrate was left to stand as it was at room temperature for 3 days fordrying thereby to form a stainproof layer, and a laminate in the form ofan optical recording medium was prepared.

With respect to the stainproof layer of the obtained laminate in theform of an optical recording medium, the test on stainproof propertiesand the test on adhesion were carried out. The results are shown inTable 1.

Example 2

A laminate in the form of an optical recording medium was prepared inthe same manner as in Example 1 except that the composition β for thestainproof layer (coating composition containing the component B)prepared in the step (g) was employed instead of the composition α forstainproof layer.

With respect to the stainproof layer of the obtained laminate in theform of an optical recording medium, the test on stainproof propertiesand the test on adhesion were carried out. The results are shown inTable 1.

Comparative Example 1

A laminate in the form of an optical recording medium was prepared inthe same manner as in Example 1 except that the composition containingno silica for light transmitting layer prepared in the step (e) wasemployed instead of the silica-containing composition for lighttransmitting layer.

With respect to the stainproof layer of the obtained laminate in theform of an optical recording medium, the test on stainproof propertiesand the test on adhesion were carried out. The results are shown inTable 1.

Comparative Example 2

A laminate in the form of an optical recording medium was prepared inthe same manner as in Example 1 except that no stainproof layer wasformed.

With respect to the stainproof layer of the obtained laminate in theform of an optical recording medium, the test on stainproof propertiesand the test on adhesion were carried out. The results are shown inTable 1. TABLE 1 Stainproof properties Pure water Hexadecane contactangle contact angle (°) (°) Adhesion Example 1 105 65 ◯ Example 2 109 61◯ Comparative 105 64 X Example 1 Comparative 80 30 — Example 2

When Examples 1 and 2 are compared with Comparative Example 2, it isfound that the laminates in the form of an optical recording medium ofExamples 1 and 2 are much excellent in stainproof properties as comparedwith the laminate of Comparative Example 2. Accordingly, it is confirmedthat a laminate in the form of an optical recording medium having alight transmitting layer formed by curing the composition A and astainproof layer containing the component B is excellent in stainproofproperties.

Further, when Examples 1 and 2 are compared with Comparative Example 1,it is found that the laminates in the form of an optical recordingmedium of Examples 1 and 2 are excellent in adhesion of the stainprooflayer as compared with the laminate of Comparative Example 1.Accordingly, it is confirmed that a laminate in the form of an opticalrecording medium having a light transmitting layer formed by curing thecomposition A and a stainproof layer containing the component B isexcellent in adhesion of the stainproof layer to the light transmittinglayer.

INDUSTRIAL APPLICABILITY

The present invention is widely applicable to optional fields of opticalrecording media.

Particularly, it is very suitably used for CD, CD-R, CD-RW, DVD, opticalrecording media suitable for a blue laser, etc.

The present invention has been described in detail with reference tospecific embodiments, but it is obvious for the person skilled in theart that various changes and modifications are possible withoutdeparting from the intention and the scope of the present invention.

The present invention is based on Japanese Patent Application filed onNov. 11, 2004 (JP-2004-327999), and the entire disclosure thereofincluding specification, claims, and summary are incorporated herein byreference in its entirety.

1. An optical recording medium, which comprises a substrate, a recordingand retrieving layer formed on the substrate, a light transmitting layerformed by curing the following component A, formed on the recording andretrieving layer, and a stainproof layer containing the followingcomponent B, bolded on the light transmitting layer: component A: a coposition containing silica particles and an oligomer having a urethanebond, capable of being cured by irradiation with radiation, component B:an alkoxysilane compound containing a fluorine atom and/or a hydrolysateof the alkoxysilane compound.
 2. The optical recording medium accordingto claim 1, wherein the silica particles contained in the component Aare colloidal silica, or silica particles of a hydrolysate of anoligomer of an alkoxysilane.
 3. The optical recoding medium according toclaim 1, wherein the silica particles contained in the component A havea number-average particle size of at least 0.5 nm and at most 50 nm. 4.The optical recording medium according to claim 1, wherein the silicaparticles contained in the component A are surface treated by a silanecoupling agent.
 5. The optical recording medium according to claim 1,wherein the alkoxysilane compound containing a fluorine atom containedin the component B is a silane coupling agent containing a fluoroalkylgroup or a fluoroaryl group.
 6. A process for producing the opticalrecording medium as defined in claim 1, which comprises a step of curingthe component A on the recording and retrieving layer to form the lighttransmitting layer, and a step of applying a composition containing thecomponent B and a solvent and having a solid component in an amount ofat least 0.01 wt. % and at most 1 wt. % to the light transmitting layerand drying the composition to form the stainproof layer.
 7. The processfor producing the optical recording medium according claim 6, wherein inthe step of forming the light transmitting layer, the silica particlesare prepared in a liquid medium containing a solvent, the oligomerhaving a urethane bond is dissolved in the liquid medium, and thesolvent in the liquid medium is removed to prepare the component A. 8.The process for producing the optical recording medium according toclaim 6, wherein the solid component includes the alkoxysilane compoundcontaining a fluorine atom and/or the hydrolysate of the alkoxysilanecompound.
 9. The process for producing the optical recording mediumaccording to claim 6 wherein the solvent is a halogen organic solvent.